Color correction to compensate for displays&#39; luminance and chrominance transfer characteristics

ABSTRACT

Displays are provided with circuitry performing color correction to compensate for the displays&#39; luminance and chrominance transfer characteristics. Some techniques are suitable for RGBW displays and for subpixel-rendered displays. Some displays include an external light source (e.g. a backlight unit in LCDs), and the color correction is coordinated with dynamic control of the light source.

BACKGROUND OF THE INVENTION

The present invention relates to image processing, and more particularlyto color correction for displaying an image.

In digital image processing, an image is typically represented as anumber of pixels. Each pixel's color is defined by the color'scoordinates in some color space, e.g. sRGB. The display converts thecolor coordinates to “grey levels” which are then used to defineelectrical signals (e.g. voltages) that determine luminous states ofcorresponding areas on the screen of the display. (Sometimes, the colorcoordinates themselves can be used as grey levels.) The luminancedisplayed in the screen area is some function (called “gamma function”,“gamma transfer function”, “gamma transfer characteristic”, or “gammacurve”) of the corresponding grey level.

For many displays, the gamma function is non-linear, and can beapproximated by a power relationship:

L=v^(γ)  (1)

where L is the luminance, v is the grey level, and γ is a constant forthe display. In many CRT (Cathode Ray Tube), LCD (Liquid CrystalDisplay), and some other types of devices, γ is about 2.2.

For many displays, the relationship (1) is only approximate, and can becorrected using look-up tables (LUTs) containing experimental dataobtained for a given display and stored in the display.

A typical color display is associated with a set of primary colors, e.g.red, green and blue. The display accepts separate grey levels for eachof the primary colors (i.e. for each “channel”). The gamma functions canbe different for different channels, and therefore separate LUTs can beprovided for each channel.

For example, a color LCD may include a number of red, green and bluesubpixels. The subpixels have identical liquid crystal cells, but havecolor filters of different colors (red, green, blue). The liquid crystalcells however have different optical activity with respect to wavelengthand hence to color. Depending on the spectral bandwidth of the colorfilters, such optical activity can result in unequal luminance gammatransfer characteristics amongst the R, G, B channels. In addition, theoptical activity can result in chrominance deviations (e.g. huedeviations) within each channel. The unequal gamma transfercharacteristics can be corrected using the separate LUTs for eachchannel. The chrominance deviations have not been considered a seriousproblem for the RGB LCDs due to the narrow spectral bandwidth of thecolor filters.

SUMMARY

This section summarizes some features of the invention. Other featuresmay be described in the subsequent sections. The invention is defined bythe appended claims, which are incorporated into this section byreference.

Any color correction performed to adjust the color data to the transfercharacteristic of a display will be called herein “automatic colorcorrection” or ACC. As used herein, the term “transfer characteristic”is a general term which covers both the gamma transfer characteristic(i.e. luminance characteristics) and chrominance characteristics (e.g.chrominance shift of the white subpixels).

In RGBW displays, the white subpixels have a high spectral bandwidth,and the chrominance deviations in the white channel can be significant.The chrominance deviations in the white channel can be compensated byadjusting luminances of the R, G and B grey levels, as described in U.S.patent application published as US 2008/0252797 A1 on Oct. 16, 2008,filed by Hamer et al., incorporated herein by reference. This patentapplication describes white chrominance correction for OLED (organiclight emitting diode) displays, and states that the invention of thatpatent application can be applied to LCDs. The OLED display has red,green blue and white emitters. First, experiments are performed tomeasure the white chrominance deviations. More particularly, the whiteemitters are turned on at different intensities, and for each intensitythe corresponding color actually displayed is measured and recorded inlinear RGB coordinates. In this manner, three curves are obtained R(W),G(W), B(W) which provide the linear R, G, and B coordinates for each Wvalue. These three curves, and the corresponding inverse functions W(R),W(G), W(B), are recorded as tables and stored in the display.

The display receives image data in some color space such as sRGB, andconverts such data to linear RGB. Then the linear RGB data are convertedto RGBW in such a way as to correct the white chrominance. Moreparticularly, let us denote a pixel's linear RGB coordinates as Rp, Gp,Bp. The display calculates the corresponding RGBW coordinates Rw, Gw,Bw, Ww as follows. The white coordinate Ww is set to the minimum ofW(Rp), W(Gp), W(Bp), or to some number below the minimum. (A numberbelow the minimum can be used in order to obtain greater spatialuniformity of luminance in displaying near-white colors.) Thecoordinates Rw, Gw, Bw are obtained by subtracting the values R(Ww),G(Ww), B(Ww) from the corresponding RGB coordinates, i.e.:

Rw=Rp−R(Ww)

Gw=Gp−G(Ww)

Bw=Bp−B(Bw)

OLED displays are also addressed by U.S. patent application published asUS 2006/0262053 A1 on Nov. 23, 2006, filed by Lee et al.

In some embodiments of the present invention, ACC is performed bydetermining a suitable correction of the color coordinates (such asRGBW) for each combination of values of the color coordinates. Forexample, suitable tables can be generated which provide correctedcoordinates for each combination of the coordinate values. However, suchtables can be quite large. If computational circuitry is used in lieu ofthe tables, the circuitry can be complex. Therefore, in someembodiments, the following techniques are used. In some RGBWembodiments, the white chrominance correction is calculated in terms ofthe RGB coordinates. Also, conventional tables can be used to correctthe luminance of the red, green, blue and white colors. If an inputcolor is a shade of grey, then the white chrominance correction tableand the white luminance correction table can be used to correctlydisplay the color. If the color is fully saturated, then the whitecoordinate is zero, and the red, green and blue luminance tables providea correct display of the saturated color. If the input color is neitherwhite nor fully saturated, then interpolation is used between (i) thewhite color defined by the white coordinate and (ii) the fully saturatedcolor defined by the RGB coordinates.

Further, the inventors have observed that in RGBW displays, colorcorrection presents a special challenge if the display uses adynamically controlled light source. For example, LCD subpixels do notemit light, so the LCD may include a backlight panel containing aseparate light source 110 (FIG. 1) which illuminates the subpixels inscreen unit 120. Light source 110 is located behind the subpixels. InLCDs, screen unit 120 is sometimes called an LCD panel, and it containsscreen driver circuitry 124 which drives the subpixels (not shown) inscreen 126. The light source 110 can be dynamically controlled (i.e.controlled based on the image displayed) to reduce power consumption. Ifthe image is dark, the light source is operated at a lower output power,and the subpixels are made more transmissive to compensate for the loweroutput of the light source. Making the subpixels more transmissive isachieved by increasing the subpixel values (the grey levels). Forexample, if the sRGB input data are converted to a linear RGBWrepresentation (by block 130 of FIG. 1), and the light source power isreduced by some factor BL (i.e. is multiplied by the factor BL<1 asdetermined by block 140), then the RGBW values can be multiplied by theinverse of BL (denoted INVy and determined by block 140). In FIG. 1, themultiplication is performed by scaling block 150. The scaled RGBW valuesare then converted to grey levels (denoted Rg, Gg, Bg, Wg for therespective red, green, blue and white subpixels) by “output gamma” block160. Dynamic control of the light source is sometimes called DynamicBacklight Control or DBLC.

Some embodiments of the present invention correct the white chrominanceand the RGB luminance while performing RGBW conversion 130. However, thecorrection may be invalid for the scaled RGBW values (the output ofblock 150). Therefore, image degradation can be high. Other embodimentsperform the color correction on the scaled data (the output of block150). However, once the scaled data are color-corrected, the scalingparameters BL, INVy may be non-optimal, resulting in unnecessarily highpower consumption and/or brightness distortion. In some embodiments,various techniques are used to provide low image degradation and lowpower consumption. For example, in some embodiments, the interpolationtechnique described above can be incorporated into output gamma 160.

Another challenge is presented in subpixel-rendered (SPR) displays,regardless of whether or not the displays use an external light source.In non-SPR displays, each image pixel is mapped into a separate set ofsubpixels which can display any primary color. In SPR displays, toreduce the number of the subpixels, a pixel can be mapped into asubpixel area which lacks one or more primary colors. Also, or in thealternative, a subpixel of some primary color may be shared by multiplepixels. In such cases, a subpixel's value may be determined frominformation on multiple pixels in a subpixel rendering (SPR) operation.The inventors have discovered that some ACC techniques may work wellwhen applied to the subpixel values generated by the SPR operation.Surprisingly, some ACC techniques work well when applied to the RGBWdata before the SPR operation. This is surprising because the SPRoperation changes the RGBW data and hence may invalidate the colorcorrections performed before the ACC.

Some embodiments include novel methods of constructing the ACC tables toreduce the table size and increase the maximum image brightnessavailable for many images. Increased brightness is highly valued inportable, battery-operated devices such as mobile telephones, but theinvention is not limited to such devices.

This summary is just a brief introduction to some features of theinvention. The invention is not limited to RGBW displays, but coversdisplays using other primaries, e.g. cyan. The invention is not limitedto the features and advantages described above except as defined by theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a display according to prior art.

FIGS. 2 and 3 are block diagrams of displays according to someembodiments of the present invention.

FIG. 4 illustrates a prior art mapping of an image consisting of pixelsinto a display with subpixels.

FIGS. 5-14 are block diagrams of displays according to some embodimentsof the present invention.

DESCRIPTION OF SOME EMBODIMENTS

The embodiments described in this section illustrate but do not limitthe invention. The invention is defined by the appended claims.

Some embodiments of the present invention modify the display of FIG. 1as follows. The color correction is incorporated into the output gammablock 160. The display may or may not use DBLC, and in particular theDBLC block 140 and scaler 150 may or may not be present. Light source110 may or may not be present, and in particular the display may benon-LCD type. The display may use LUTs or computational circuitry, orcombination of the two. For example, some values can be stored in LUTs,white other values can be interpolated from the LUT values. For ease ofdescription, the examples described below use tables, but these examplesare not limiting.

FIG. 2 illustrates a block diagram of a display with such tables 220,230, 240 in output gamma block 160.

The color correction technique illustrated in FIG. 2 will be called a“vector method” herein. Output gamma block 160 receives subpixel valuesRs, Gs, Bs, Ws for the subpixels and generated the grey values RgGgBgWgfor screen driver 124. Each subpixel value corresponds to thecorresponding subpixel's luminance when the subpixel will emit thecorresponding primary color. The input values RsGsBsWs are possibly, butnot necessarily, linear with respect to luminance. In the example ofFIG. 2, the display receives sRGB data, converts them to possibly (butnot necessarily) linear RGBW coordinates RwGwBwWw, and processes thesecoordinates to generate the subpixel values RsGsBsWs by digitalprocessing block 210. The invention is not limited to sRGB or anyparticular kind of processing in block 210. Block 210 may include ascaler 150 or a DBLC block 140 as in FIG. 1, and/or an SPR renderingunit, and/or some other units, or block 210 may be absent (i.e. theRsGsBsWs data can be identical to RwGwBwWw). The display may or may notbe an SPR display.

Output gamma block 160 has ACC tables “ACC0” for luminance correction,and “ACC1” for white chrominance correction. The ACC0 tables 220, 230include a separate table for each primary color. The ACC0 tables 220 arethe three tables for the red, green and blue primaries, and the ACC0table 230 is the table for the white primary. The RGB tables 220 can beconventional tables used in prior RGB displays. The grey values storedin these tables are shown as Rg0, Gg0, Bg0. For example, for each redvalue Rs, the ACC0 table 220 for the red primary returns thecorresponding grey value Rg0. The green and blue ACC0 tables aresimilar. Tables 220 can be constructed by conventional techniques. Forexample, for the red primary, the gamma function for the display isconstructed by turning on just the red pixels (i.e. supplying theRgGgBgWg values to screen driver 124 with Gg=Bg=Wg=0), and measuring theluminance of the image on screen 126. The gamma function is inverted toobtain the ACC0 table for the red primary. The green and blue ACC0tables are constructed in a similar manner.

White ACC0 table 230 returns a white grey value Wg for each input whitesubpixel value Ws. Table 230 can be constructed in the same way astables 220.

ACC1 table 240 converts a white subpixel value Ws into a triple of RGBgrey values Rg1, Gg1, Bg1 which represent the red, green and blue greyvalues needed to compensate for the white chrominance shift present whenthe white grey value Wg from white ACC0 table 230 is supplied to screenunit 120.

For fully saturated colors (i.e. for colors having at least one sRGBcoordinate equal to zero), Ws will typically be zero, and the RGB ACC0tables 220 will provide correct grey values Rg0, Gg0, Bg0 which can bepassed unchanged by interpolation block 250 (described below). Thesevalues (shown as RgGgBg at the output of interpolation circuit 250) arepassed to screen unit 120 through dither block 270. When Ws=0, whiteACC0 table 230 returns zero grey value Wg. Thus, the color correction inblock 160 works well for fully saturated colors.

Interpolation block 250 performs interpolation between the RgGgBgoutputs of tables 220, 240 based on a saturation parameter Sat output byblock 260 and indicative of the color saturation. For example, in someembodiments,

Sat=Ws/max(Rs,Gs,Bs,1)  (2A)

where “1” is added on the right to avoid division by zero. In this case,the Sat parameter indicates the inverse of the saturation. In someembodiments, the subpixel values Rs, Gs, Bs, Ws are weighted withrelative luminances. For example, let Y_(r) denote the luminance ofscreen 126 when the red subpixels are maximally emissive and all thegreen, blue and white subpixels are minimally emissive. The Y_(g),Y_(b), Y_(w) be defined in a similar manner for the green, blue, andwhite subpixels. Then in some embodiments:

Sat=(Ws*Y _(w))/max[(Rs*Y _(r)),(Gs*Y _(g)),(Bs*Y _(b)),1]  (2B)

Interpolation in block 250 can be linear or some other kind. Forexample, in some embodiments, linear interpolation is used, and Satchanges from 0 to 1 inclusive. For example, in the case (2A), the RGBWconversion 130 is performed so Ww does not exceed max(Rw, Gw, Bw), anddigital processing 210 is such that Ws does not exceed max(Rs, Gs, Bs).Consequently, Sat is at least 0 and at most 1. In some such embodiments,denoting the block 250 output as RgGgBg:

Rg=Sat*Rg1+(1−Sat)*Rg0

Gg=Sat*Gg1+(1−Sat)*Gg0

Bg=Sat*Bg1+(1−Sat)*Bg0  (3)

In some embodiments, if the color is white or grey, and Sat is definedby (2A), then Rs=Gs=Bs=Ws, so Sat=1. Therefore, in case of (3), theinterpolation output (Rg, Gg, Bg)=(Rg1, Gg1, Bg1), i.e. the white orgrey color display will be accurate.

Polynomial or some other kind of interpolation can also be used, or someother kind of combination of the outputs of tables 220, 240. Forexample, in some embodiments,

Rg=(Sat*Rg1^(2.2)+(1−Sat)*Rg0^(2.2))^(1/2.2)

Bg=(Sat*Gg1^(2.2)+(1−Sat)*Gg0^(2.2))^(1/2.2)

Rg=(Sat*Bg1^(2.2)+(1−Sat)*Bg0^(2.2))^(1/2.2)

Of note, even though Sat may be thought of as the inverse of thesaturation, the value Sat for a given color depends on the choice of thecolor's RGBW representation Rs, Gs, Bs, Ws (i.e. the choice of metameras described below). As is well known, for most colors, the RGBWrepresentation is not unique. The value Sat is thus indicative of therelative magnitude of the Ws coordinate versus the Rs, Gs and Bscoordinates.

In some embodiments, in order to reduce the circuit size (the gatecount), the interpolation is performed on some but not all of the RGBgrey values. For example, the red and blue grey values Rg, Bg can beobtained as in equations (3), but the green grey value Gg can be set toGg1 or Gg0.

In some embodiments, the interpolation can also be performed on thewhite grey values. For example, tables 220 can be modified to provide awhite grey value Wg0 as appropriate for each set of the RsGsBsWs values.Interpolation block 250 can generated Wg by interpolating between theWg0 value and the value Wg generated by table 230. These examples arenot limiting.

Dithering block 270 is optional, and can be used when the RgGgBgWgoutput of output gamma 160 must be truncated. For example, in someembodiments, the grey values accepted by screen unit 120 are 8-bit wide,but the grey values generated by output gamma 160 are 10-bit wide.Spatial, temporal, or spatial-temporal dithering can be performed usingknown techniques, and other truncation techniques can also be used.

In some embodiments, the white ACC0 table 230 is constructed to at leastapproximate the inverse of the gamma function of screen unit 120 for thewhite subpixels.

More particularly, in some embodiments, the ACC0 and ACC1 tables areconstructed as follows.

A set of optical measurements is taken of the display by supplyingappropriate grey values Rg, Gg, Bg, Wg to screen unit 120. Moreparticularly:

1. The chromaticity and the luminance of the brightest R, G, B and Wprimaries are obtained. More particularly, for one of the primaries, thecorresponding grey value Rg, Gg, Bg, or Wg at the input of screen unit120 is set to maximum value (e.g. 255), and the remaining grey valuesare zero for the entire screen; if a backlight unit is used, its outputpower is maximized. The chromaticity (e.g. in CIE xy coordinates) andthe luminance are then measured. In some embodiments, for the R, G, andB coordinates, the radiance is measured instead of the luminance becauseluminance measurements are sometimes too noisy.

2. For each of the four primaries, the primary's grey value is rampedfrom 0 to the maximum value while the other primaries' grey values areheld at zero. For each value in the ramp, the primary's luminosity ismeasured. In some embodiments, the grey values are 8-bit wide, and eachprimary is ramped by being set to 0, 8, 16, . . . , 240, 248, 255 (33points).

3. For the W ramp, the chromaticity is recorded for each Wg value in CIExy coordinates.

For example, when these measurements were performed on a sample DID(Digital Information Display) LCD panel, the following results wereobtained. The CIE xy measurements on the brightest primaries were:

R G B W x 0.633 0.287 0.156 0.2815 y 0.328 0.623 0.051 0.2657 z 0.0390.09 0.793 0.4528

The luminosity measurements of the brightest primaries, when normalizedto the sum of 1.0, were as follows:

R 0.062542 G 0.245172 B 0.038776 W 0.653509

These values were used with well known colorimetric algebra to generatethe following two matrices. The first matrix converts normalized RGBWmeasurements to CIE XYZ coordinates. This matrix was:

$\quad{\begin{matrix}0.120698 & 0.112945 & 0.118610 & 0.692371 \\0.062542 & 0.245172 & 0.038776 & 0.653509 \\0.007436 & 0.035418 & 0.602935 & 1.113696\end{matrix}}$

The second matrix, denoted “R2X” below, consists of the first threecolumns of the first matrix and converts linear RGB to CIE CYZ:

$\quad{\begin{matrix}0.120698 & 0.112945 & 0.118610 \\0.062542 & 0.245172 & 0.038776 \\0.007436 & 0.035418 & 0.602935\end{matrix}}$

The luminosity measurements of the ramped primaries can be used in knownways to construct an output gamma curve for each primary. The gammacurves can be inverted to construct the ACC0 tables 220, 230.

The ACC1 table 240 can then be constructed as follows.

1. For some range of Ws values, e.g. each possible value, the CIE XYZcoordinates of the “ideal” white color are determined. For example,suppose that Ws can vary from 0 to 255, and the ideal white color issome color DD. For example, DD may have the chromaticity of D65 and theluminance of the brightest white primary measured as mentioned above.Suppose that the output gamma for the white primary is the powerfunction (1) with γ=2.2, or a function with close values. The idealwhite XYZ coordinates, denoted as C2_(i), will then be

C2_(i) =DD*(i/255)^(2.2), where i=0, 1, 2, . . . 255  (4)

2. Let us denote the corresponding 256 coordinates of the measured whiteprimary as WXYZ_(i). These coordinates can be obtained from theluminance and chromaticity measurements of the white primary asdescribed above, when Wg=i is ramped from 0 to 255. As stated above, insome embodiments, the measurements are made only for a sub-range Wg=0,8, 16, . . . . In this case, the remaining WXYZ_(i) values can beobtained by interpolation (e.g. linear or some other type ofinterpolation).

The desired white chrominance correction for each value Wg=i can then beexpressed in CIE XYZ coordinates as:

SUM_(i) =C2_(i) −WXYZ _(i)  (5)

These colors can be converted to the linear RGB coordinates using thematrix R2X described above, to obtain:

ACC1_(i) =R2X ⁻¹SUM_(i)  (6)

Thus if Ws corresponds to the white ideal color C2_(i), i.e.

Ws=const*(i/255)^(2.2)  (7)

where “const” is a constant dependent on the bit width of Ws, thenACC1_(i) is the desired RGB correction in the linear RGB coordinatescorresponding to the display's primary RGB chromaticities. The linearRGB coordinates of the ACC1_(i) vectors are then submitted as inputs tothe RGB ACC0 tables 220, and the outputs of the RGB ACC0 tables arestored in ACC1 table 240 for the respective Ws values (7) rounded to aninteger. For the remaining Ws values, the ACC1 table entries can beinterpolated, or omitted altogether and obtained by interpolation whendisplaying an image.

In some embodiments, the choice of the white-point DD affects how wellthe white chromaticity can be corrected. If the DD chromaticity ischosen as the chromaticity of the color produced when all the red,green, blue and white emitters are maximally emissive, then the ACCcorrection will be small and the ACC will be more accurate.Alternatively, the DD chromaticity can be chose as the chromaticity ofthe color produced when the white emitters are maximally emissive andthe red, green and blue emitters are minimally emissive.

If the maximum ACC1 correction is small, then the RgGgBg output ofinterpolation block 250 is less likely to overflow or underflow,resulting in a greater range of allowable values Rg0Gg0Bg0 of ACC0tables 220, and hence in a greater range of allowable values RsGsBs atthe output of processing block 210. (If an overflow or underflow occurs,the grey value Rg, Gg, or Bg may be hard-clamped to the maximum orminimum allowable value, or more complex gamut mapping may be performedusing known techniques; see e.g. U.S. patent application published asno. 2007/0279372 A1 published on Dec. 6, 2007, filed by Brown Elliott etal., entitled “MULTIPRIMARY COLOR DISPLAY WIITH DYNAMIC GAMUT MAPPING”,incorporated herein by reference.)

FIG. 3 shows an alternative embodiment which implements a “delta offset”color correction method. Here the ACC1 table 240 stores the ACC1_(i)corrections (the “delta offsets”, denoted (dRi, dRg, dBi) in FIG. 3),i.e. the corrections are in the linear color space. The ACC1_(i)correction (dRi, dRg, dBi) is added to the respective linear RsGsBscoordinates generated by digital processing block 210. The addition isperformed by adder 310. The adder's output is fed to the RGB ACC0 tables220 constructed as in FIG. 2. The RgGgBg output of tables 220 isprovided at the output of output gamma 160. The Wg grey values aregenerated by table 230 as in FIG. 2. The circuitry performing the linearcolor correction (i.e. the ACC1 table 240 and the adder 310) is shown asa separate block 320, but the invention is not limited to any particulardivision of circuitry into blocks.

In some embodiments, the table 240 generates also a correction to thewhite value Ws. Alternatively, a white-value correction can be generatedfrom the RsGsBs coordinates.

It is conventional in some dark RGBW displays, or in RGBW displays usedin unfavorable viewing conditions, to increase image brightness byincreasing just the white subpixels' values. In such displays, forexample, the white subpixel value Ws is multiplied by a “white gain”factor, and the result is hard-clamped to the allowable range of the Wsvalues. (As used herein, “hard-clamping” a value to a range of somenumbers A to B means setting the value to the low bound A if the valueis below A, and setting the value to the high bound B if the value isabove B.) In FIGS. 2 and 3, the white gain adjustment can be done eitherbefore or after the white chrominance correction. For example, in FIG.2, the Ws value can be adjusted before being provided to output gamma160, or alternatively the white-gain adjustment can be performed on theWg value output by W ACC0 table 230 or dither block 270. In FIG. 3, thewhite-gain adjustment can be performed on: (i) the output Ws of digitalprocessing block 210; or (ii) the input Ws to W ACC0 table 230 but noton the input Ws of ACC1 table 240.

As stated before, the embodiments of FIGS. 2 and 3 can be used with SPRdisplays. If the display is non-SPR, then each pixel of the originalimage is associated with a quad of a red subpixel, a green subpixel, ablue subpixel, and a white subpixel. A chromaticity correction producedby ACC1 table 240 for a white subpixel is combined (by blocks 250, 260in FIG. 2 or adder 310 in FIG. 3) with the subpixel values for the red,green and blue subpixels of the same quad.

However, in some SPR displays, there may be no unique associationbetween a white subpixel and red, green and blue subpixels. One suchdisplay is described in PCT application published as no. WO 2006/127555A2 on 30 Nov. 2006 and U.S. patent application Ser. No. 11/278,675published as no. 2006/0244686 A1 on 2 Nov. 2006 and illustrated in FIG.4. In the example of FIG. 4, an image 404 is represented by a number ofpixels 406. Each pixel 406 is mapped into a set of only two subpixelswhich are to display the pixel's color.

More particularly, screen 126 of this display contains red subpixels420R, blue subpixels 420B, green subpixels 420G, and white subpixels420W. All these subpixels 420 are equal in area. Each pixel 406 ismapped into a subpixel pair 424 consisting of two adjacent subpixels inthe same row. Each pair 424 consists of either a red subpixel 420R and agreen subpixel 420G (such pairs are called “RG pairs” below), or eachpair consists of a blue subpixel 420B and a white subpixel 420W (“BWpair”). In each RG pair, the red subpixel is to the left of the greenone. In each BW pair, the blue subpixel is on the left. The RG and BWpairs alternate in each row and each column. This is just an exemplaryembodiment, and other subpixel arrangements are possible. For example,the red subpixel may be on the right in each RG pair, and the bluesubpixel may be on the right in each BW pair. Also, the subpixels do nothave to be equal in area. For example, in some displays, the whitesubpixel's area is adjusted to produce desirable ratios of RGB white toW white, and the white subpixel's area may be different from the area ofa non-white subpixel. The red, green and blue subpixels may also havedifferent areas.

Each pixel 406 in column x and row γ of the image (pixel “406 _(x,y)”below) is mapped into the subpixel pair 424 in column x and row γ (“424_(x,y)” below). In screen 126, the consecutive indices x and γ denoteconsecutive pairs, not consecutive subpixels. Each pair 424 has only twosubpixels, and provides a high range and resolution in luminance but notin chrominance. Therefore, part of the input pixel's luminance may haveto be shifted to adjacent pairs 424 in a subpixel rendering operation(SPR). Many subpixel rendering operations can be used. One example is asfollows. Let the RGBW coordinates of pixel 406 _(x,y) be denoted asRw_(x,y), Gw_(x,y), Bw_(x,y), Ww_(x,y). Then, if a subpixel pair 424_(x,y) is not at the edge of the display, then the red subpixel's valueRs for the pair is computed as follows:

Rs=½*Rw _(x,y)+⅛*Rw _(x−1,y)+⅛*Rw _(x+1,y)+⅛*Rw _(x,y−1)+⅛*Rw_(x,y+1)  (8)

Similar computation can be used for the green, blue and white subpixelsexcept at the edges. If a subpixel pair 424 is at an edge of thedisplay, then the pixel values behind the display edge are needed, andthey can be set to zero or some other values. See e.g. the United StatesPatent Publication No. 2005/0225563 having application Ser. No.10/821,388 and entitled “IMPROVED SUBPIXEL RENDERING FILTERS FOR HIGHBRIGHTNESS SUBPIXEL LAYOUTS”, incorporated herein by reference.

In FIGS. 2 and 3, the SPR operation can be performed by digitalprocessing block 210, and the output RsGsBsWs of the SPR operation canbe provided to output gamma 160 of FIG. 2 or Delta ACC circuit 320 ofFIG. 3. Alternatively, before being provided to block 160 or 320, theRsGsBsWs output of the SPR operation can be subjected to otherprocessing, e.g. sharpening, gamut clamping (gamut clamping is needed ifthe RsGsBsWs values are outside of the display gamut), or other types ofprocessing.

In the ACC circuitry of FIGS. 2 and 3, the RGB coordinates obtained fromACC1 table 240 for a white subpixel 420W are combined (by blocks 260,250 or adder 310) with the RGB coordinates of some adjacent subpixels420R, 420G, 420B (“correction subpixels” below) which can be chosen inany desired manner. For example, in some embodiments, the blue subpixel420B is taken from the same pair 424 as the white subpixel 420W, and thered and green subpixels 420R, 420G are taken from the pair 424immediately to the left. If however the white subpixel 420W is in a pair424 at the left edge of the screen, then the red and green correctionsare not performed, and in some embodiments the blue correction is alsonot performed. Other choices of correction subpixels are also possible.In some embodiments, a single correction value (i.e. one of Rg1, Gg1,Bg1 of FIG. 2 or of dRi, dGi, dBi of FIG. 3) may be distributed to twoor more subpixels 420 of the respective red, green or blue primarycolor.

In FIG. 5, the white chrominance correction is performed before the SPR.In this embodiment, digital processing 210 generates RwGwBwWw data, i.e.generates a data quad for each pixel 406. In delta ACC block 320, the Wwoutput of block 210 for each pixel 406 is provided to ACC1 table orcircuit 240, which generates the appropriate RGB correction ACC1r(dRi,dGi, dBi) for the pixel. The correction is added to the RwGwBwcoordinates for the pixel by adder 310. The sum, also denoted RwGwBw, isprovided to SPR block 510 together with the Ww output of block 210. SPRblock 510 performs subpixel rendering to generate the Rs, Gs, Bs, Wssubpixel values for the subpixels 420. Optionally, the Ws output of SPRblock 510 is multiplied by the white gain factor WG. The multiplicationis performed by multiplier 520. The subpixel data are provided to outputgamma 160, which can be as in FIG. 3 for example. The subsequentprocessing can be as in FIG. 3.

Performing white chrominance correction before SPR as in FIG. 5 isadvantageous because before SPR, the association between the Ww valueand the Rw, Gw, Bw values is well defined by the pixels 406.

In some embodiments, ACC1 table 240 for the embodiment of FIG. 5 isgenerated as follows. Some ideal white color DD is chosen, possibly asdescribed above in connection with FIGS. 2 and 3. The white colormeasurements WXYZ_(i) are performed for each grey value Wg=i (e.g. i=0,1, . . . , 255 in case of 8-bit grey values), with Rg=Gg=Bg=0. For eachvalue Ww, the output of ACC1 table 240 is determined as the differencebetween the ideal color DD*Ww/max{Ww} and a measured color WXYZi, wherethe measured color WXYZi is chosen as the measurement whose luminance(e.g. the CIE Y coordinate) is the closest to the luminance of Ww. Thevector difference is expressed in the linear RGB coordinates using thematrix R2X⁻¹ described above.

In some embodiments, ACC1 table 240 is constructed as follows. A numberof measurements are performed to obtain CIE xyY coordinates. In eachmeasurement, one of the primaries' grey values is positive and theremaining grey values are zero (as in step 1 discussed above for theembodiment of FIG. 2). For each primary, for each of the x, y, and Ycoordinates, the measurements are smoothened by a box filter. Also, foreach primary, the Y measurements are normalized by subtracting thesmallest Y value and dividing the results by a suitable factor so thatthe normalized Y values add up to 1.

Let mx denote the number of measurements performed for each primary. Forease of discussion, it will be assumed that the grey values are 8-bitwide, and m×=256 measurements are performed for each primary. If i is agrey value, then let us denote the smoothed and normalized values as forthe red primary as Rx_(i), Ry_(i), RY_(i), and use similar notation forthe other primaries.

Then the smoothed and normalized values for the brightest primaries(i.e. for the grey values of 255) are used to obtain conversion matricesbetween the RGBW color space tied to the brightest measured primariesand the CIE YXZ color space. More particularly, the following matrix isdefined:

$\mspace{20mu} {p = {\begin{matrix}{Rx}_{mx} & {Gx}_{mx} & {Bx}_{mx} & {Wx}_{mx} \\{Ry}_{mx} & {Gy}_{mx} & {By}_{mx} & {Wy}_{mx} \\{1 - {Rx}_{mx} - {Ry}_{mx}} & {1 - {Gx}_{mx} - {Gy}_{mx}} & {1 - {Bx}_{mx} - {By}_{mx}} & {1 - {Wx}_{mx} - {Wy}_{mx}}\end{matrix}}}$

Then each element of p is multiplied by the corresponding Y/y value(where Y is normalized and y is smoothed). The resulting matrix, denotedM2X below, provides conversion from RGBW to XYZ. The first three columnsof M2X form a matrix, denoted by R2X, that converts from RGB to XYZwhere RGB is the linear color space defined by the brightest red, greenand blue primaries of the display. Both R2X and M2X are “normalized” inthe sense that the brightest primaries are converted to a representationwhose Y values are normalized.

Ideally, the x and γ values of each primary should be independent of theprimary's grey value i. This is not however the case for some displaysas discussed above.

The measurements Rx_(i), Ry_(i), RY_(i), etc. are then “re-sampled” tomake them more evenly spaced in a linear color space. More particularly,the grey values i are non-linear with respect to luminance, so the greyvalues are not evenly spaced in a linear space. The re-samplingprocedure assumes the power law (1) with γ=2.2. Let us introduce avariable j that takes values in a linear space. The j values are suchthat each value i corresponds to j=(i/255)^(2.2)*MAXCOL where MAXCOL isthe maximum j value (in some embodiments, MAXCOL is the maximum of eachof Rw, Gw, Bw, Ww values output by digital processing circuit 210 andconversion circuit 130. In some embodiments, MAXOL=2047. Themeasurements described above provide the xyY coordinates for the jvalues of about (i/255)^(2.2)*MAXCOL, and these values are not evenlyspaced. Therefore, the xyY coordinates for evenly spaced j values j=0,1, 2, . . . MAXCOL are interpolated from the smoothed and normalized xyYmeasurements for the unevenly spaced j values using cubic splineinterpolation. The re-sampled values are converted to the XYZ colorspace. Let us denote the XYZ values of the red, green, blue and whiteprimaries as RXYZd_(j), GXYZd_(j), BXYZd_(j), and WXYZd_(j)respectively.

The ideal white color DD can be chosen in many ways. One possibility isto choose a color identical in chromaticity to the brightest colordisplayed when all the grey values are at the maximum of mx (i.e. 255).This color DD is:

${DRGBW} = {M\; 2\; {X \cdot {\begin{matrix}1 \\1 \\1 \\1\end{matrix}}}}$

Another possibility is to choose a color identical in chromaticity tothe brightest color displayed when the white grey values Wg=mx and theremaining grey values are all zeros. This color DD is:

${DW} = {M\; 2\; {X \cdot {\begin{matrix}0 \\0 \\0 \\1\end{matrix}}}}$

In any case, the DD color is multiplied by j/MAXCOL to obtain idealwhite colors (grey colors) for different j, where MAXCOL is the maximumj value.

The resulting colors DD*j/MAXCOL are converted into the RGB space byapplying the matrix R2X⁻¹. Let us denote the resulting ideal “ramp” asIRGB_(j) (Ideal RGB).

If DD=DRGBW, then the re-sampled RGBW data are summed up for each j tocompute RGBWXYZ_(j)=RXYZd_(j)+GXYZd_(j)+BXYZd_(j)+WXYZd_(j), and thesesums are converted to RGB using the R2X⁻¹ matrix. Let us denote theresulting values as RGBRGBW_(j). ACC table 240 is written with thedifference IRGB_(j)-RGBRGBW_(j). Each difference includes the red, greenand blue components provided by table 240 in response to Ww=j.

If DD=DW, then the re-sampled W data are converted to RGB using theR2X⁻¹ matrix. Let us denote the resulting values as WRGB_(j). ACC table240 is written with the differences IRGB_(j)-WRGB_(j). Each differenceincludes the red, green and blue components provided by table 240 inresponse to Ww=j.

In either case (DD=DRGBW or DD=DW), in some embodiments, the bluecomponents in table 240 are hard-clamped to the maximum value of zero,i.e. the blue corrections are prevented from being positive.

METAMER ADJUSTMENT. White chrominance correction involves addingpositive or negative numbers output by table 240 to red, green and bluevalues, and the resulting colors may be out-of-gamut (e.g. if theaddition causes overflow or underflow). The out-of-gamut colors can bebrought inside gamut by a suitable gamut clamping operation, but gamutclamping may lead to image distortion. Image distortion can be reducedby selecting a suitable RGBW representation in block 130 or 320 sincecolor representation in RGBW coordinates is typically not unique. Forexample, if Rw, Gw, or Bw coordinate overflows, the RwGwBw coordinatesmay be reduced via increasing Ww. In case of underflow, the RwGwBwcoordinates may be increased via reducing Ww.

Different RGBW representations of a color are called “metamers” in someliterature and herein. (Other literature uses the word “metamers” todenote electromagnetic waves of different spectral power distributionsperceived as the same color, but different RGBW representations do notnecessarily mean different spectral power distributions.)

FIG. 6 shows an embodiment similar to FIG. 5 but with block 320containing a metamer correction circuit 604 (“post-color correction” orPCC) at the output of adder 310. Table 240 can be constructed as in FIG.5. SPR block 510 may or may not be present (i.e. the embodiment issuitable for both SPR and non-SPR displays).

FIG. 6 represents RGBW conversion block 130 as two blocks 130.1, 130.2.Block 130.1 converts sRGB data to a linear RGB color space. This colorspace may be independent of the display, or may have primary colorshaving the same chromaticity as the red, green and blue emitters ofscreen unit 120. Block 130.1 is sometimes referred to as “input gamma”In some embodiments, all processing between input gamma 130.1 and outputgamma 160 is performed in linear coordinates (RwGwBwWw and RsGsBsWs arelinear representations of color).

Block 130.2 converts the linear RGB data (denoted r, g, b) to RGBW data(denoted Rw, Gw, Bw, Ww) using a well know method. In some embodiments,

r=M ₀ ·Rw+M ₁ ·Ww

g=M ₀ ·Gw+M ₁ ·Ww

b=M ₀ ·Bw+M ₁ ·Ww  (9)

where M₀ and M₁ are constants corresponding to the luminancecharacteristics of pixels 420 as follows:

M ₀=(Y _(r) +Y _(g) +Y _(b))/(Y _(r) +Y _(g) +Y _(b) +Y _(w))

M ₁ =Y _(w)/(Y _(r) +Y _(g) +Y _(b) +Y _(w))  (10)

where Y_(r),Y_(g), Y_(b), Y_(w) are defined as follows. Y_(r) is theluminance of display 110 when the red subpixels 420R are maximallyemissive and all the remaining subpixels are minimally emissive. Thevalues Y_(g), Y_(b), Y_(w) are defined in a similar manner for thegreen, blue, and white subpixels.

If the Ww coordinate is known, then Rw, Gw, and Bw can be computed from(9).

In some embodiments, each of the r, g, b, Rw, Gw, Bw, Ww is an integerallowed to vary from 0 to some maximum number MAXCOL inclusive. Forexample, in some embodiments, MAXCOL=2047.

The equations (9) clearly require that if r, g, or b is zero, then Wwmust be zero. If r=g=b=MAXCOL, then Ww=MAXCOL. However, for many colors,Ww can be chosen in a number of ways (to define one or a number ofmetamers). In order for each of Rw, Gw, Bw, Ww to be in the range of 0to MAXCOL inclusive, Ww must be in the following range:

minW≦W≦maxW  (11)

where

minW=[max(r, g, b)−M₀*MAXCOL]/M₁

maxW=min(r, g, b)/M₁

Table 1 below illustrates pseudocode which can be used as simulationcode for one embodiment of the ACC/PCC block 320 of FIG. 6. Thesimulation code is written in the well known programming language LUA.This language is similar to C.

At line ACCT of Table 1, the values Rw, Gw, Bw, Ww are fetched for thepixel 406 being processed. At line ACC9, the Ww value is multiplied bythe white gain factor. The resulting white-gain-corrected value is shownas Wg, and this should not be confused with the grey value Wg mentionedelsewhere. At lines ACC10-ACC12, the white-gain-corrected value Wg issupplied to ACC1 table 240. The resulting values dRi, dGi, dBi are shownrespectively as Dr, Dg, Db. At lines ACC13-ACC16, adder 310 performs theaddition to generate the sums R_pcc, G_pcc, B_pcc. The white value W_pccin line ACC16 is simply the original value Ww. The values R_pcc, G_pcc,B_pcc, W_pcc are then metamer adjusted by circuit 604 as shown in linesACC24-ACC36.

More particularly, it is first determined if any of R_pcc, G_pcc, B_pccis negative. The minimum of these three values is shown as minRGB_pcc inline ACC24. If this minimum is negative, then a positive adjustmentneeds to be made, i.e. a positive number pos_adj_pcc should be added toR_pcc, G_pcc, and B_pcc. If the minimum is not negative, thenpos_adj_pcc is set to zero. See lines ACC24-ACC28.

The white coordinate W_pcc should be reduced by an amount W_adj_pcccorresponding to pos_adj_pcc. If M₀=M₁=½, then W_adj_pcc=pos_adj_pcc.More generally, as seen from equations (9), W_adj_pcc=M₀/M₁*pos_adj_pcc.The value M₁/M₀ is often denoted as M₂.

However, the white coordinate W_pcc (i.e. Ww) should not be reduced to avalue below zero. Therefore, in line ACC31, the decrement valueW_adj_pcc is hard-clamped to its maximum possible value, i.e. to Ww. Inline ACC32, the adjusted increment RGB_adj_pcc for the red, green andblue coordinates is computed from W_adj_pcc. In lines ACC33-ACC36, themetamer adjustment is performed by incrementing R_pcc, G_pcc, B_pcc withRGB_adj_pcc and decrementing W_pcc. In lines ACC39-ACC43, the new valuesR_pcc, G_pcc, B_pcc W_pcc are hard-clamped to the maximum value MAXCOL.

TABLE 1 Delta ACC/PCC block 320ACC1.--********************************************* ACC2.     --DeltaACC and Metamer Adjustment: do it   after Post Scale and before SPRACC3.if ACCmode==4 then ACC4.  next=“sacc” ACC5. spr.create(next,xsiz,ysiz,6,2) ACC6. spr.loop(xsiz,ysiz,1,1,function(x,y) ACC7.   localRw,Gw,Bw,Ww,Lw,Ow=spr.fetch(pipeline,x,y) ACC8.     --white gain movesfrom end of SPR to here   when ACCmode==4 ACC9.   Wg =math.floor(Ww*(WG+1)/256)   --white gain   can decrease white hereACC10.   local Dr = accRdel[Wg]  --RGB delta values   are fetchedACC11.    local Dg = accGdel[Wg] ACC12.    local Db = accBdel[Wg]ACC13.   R_pcc = Rw+Dr        --the delta   values are added to thelinear RGB values ACC14.   G_pcc = Gw+Dg ACC15.   B_pcc = Bw+DbACC16.   W_pcc = Ww ACC17. ACC18. ACC19.   -- After the adding the deltaACC data, the blue   value may go negative ACC20.  -- Instead of forcingthe blue value to zero, add   some RGB component and subtract W toadjust the metamer   without clipping ACC21.  PCC_ADJ = PCC_ADJ or 0 --(OPTIONAL)   POST-COLOR-CORRECTION METAMER ADJUST ACC22. ACC23.   ifPCC_ADJ == 1 then ACC24.    minRGB_pcc = math.min(R_pcc, G_pcc, B_pcc)ACC25.    if minRGB_pcc < 0 then ACC26.     pos_adj_pcc = −minRGB_pcc --reverse   the negative sign and store it as a positive adjustmentACC27.    else ACC28.     pos_adj_pcc = 0 ACC29.    endACC30.    W_adj_pcc  = math.floor(M2_inv/32 *   pos_adj_pcc)ACC31.    W_adj_pcc  = math.min (Ww, W_adj_pcc)   -- positive RGBadjustment is ultimately limited by the   amount of W availableACC32.    RGB_adj_pcc = math.floor(M2_reg/256 *   W_adj_pcc)ACC33.    R_pcc = R_pcc + RGB_adj_pcc -- push up the   RGB component sothat post color corrected metamer   isn't negative ACC34.    G_pcc =G_pcc + RGB_adj_pcc -- ACC35.    B_pcc = B_pcc + RGB_adj_pcc ACC36.W_pcc = Ww − W_adj_pcc ACC37.   end ACC38. ACC39.   Rw =math.max(0,math.min(MAXCOL,R_pcc)) --   they are clamped inside thepipeline range ACC40.   Gw = math.max(0,math.min(MAXCOL,G_pcc))ACC41.   Bw = math.max(0,math.min(MAXCOL,B_pcc)) ACC42.   Ww =math.max(0,math.min(MAXCOL,W_pcc))ACC43.   spr.store(next,x,y,Rw,Gw,Bw,Ww,Lw,Ow) END OF TABLE 1

The embodiment of Table 1 uses a hard clamp to correct for overflow(lines ACC39-ACC42), but in other embodiments an overflow is correctedsimilarly to the underflow. More particularly, if there is an overflowbut no underflow, then the maximum overflow in coordinates Rw, Gw, Bwoutput by table 240 can be determined, and the corresponding Wwincrement can be determined to reduce or eliminate the maximum overflow.

The embodiments described above can be used with or without SPR and withor without DBLC. Some embodiments will now be described that are usefulfor displays with DBLC.

FIG. 7 shows an LCD DBLC embodiment in which SPR 510 is performed beforeACC. The ACC1 correction is performed in output gamma block 160 as inFIG. 2, but can be performed in before output gamma 160 as in block 320of FIG. 3, 5, or 6 (if ACC is performed as in FIG. 5 or 6, then ACC canbe performed before SPR 510). DBLC block 140 and scaler 150 may operateas in FIG. 1. SPR 510 is performed on the output of scaler 150.

As described above, the ACC correction may interfere with DBLC becausethe factors BL, INVy are computed based on pre-ACC data. The embodimentof FIG. 8 addresses this problem as described below. This embodiment issuitable for both SPR and non-SPR displays; SPR can be performed beforeor after ACC/PCC block 320; block 320 can be as in FIG. 6, or can skipthe metamer adjustment as in FIG. 3 or 5.

Referring first to FIG. 1, it is conventional to delay the BL and INVyvalues by one frame. More particularly, the INVy value determined fromthe sRGB data for one frame (“current frame”) can be used by scaler 150for scaling the next frame. The BL value determined from the currentframe can be used to control light source 110 when screen unit 120displays the next frame. The current frame is scaled and displayed usingthe BL and INVy values determined from the previous frame of data. Let“k” denote a frame number, and BL_(k) and INV_(k) denote the BL and INVyvalues computed from the data for frame k. Then the values BL_(k) andINV_(k) are used to scale and display the next frame k+1. This one-framedelay allows one to start displaying the current frame k before thevalues BL_(k) and INVy_(k) have been determined. In fact, displaying thecurrent frame may begin even before all the sRGB data for the currentframe have been received. To reduce image errors, the BL values can be“decayed”, i.e. the BL_(k) value can be generated by block 430 as aweighted average of the BL value determined from the data for thecurrent frame and the previous BL value BL_(k−1). In this case, theinverse values INVy will also be decayed. In some displays which display30 frames per second, when the image brightness abruptly changes, it maytake about 36 frames for the BL and INVy values to catch up with theimage brightness. This delay is acceptable in many applications. Indeed,in the absence of abrupt changes of the image brightness, the BL andINVy values typically do not change much from frame to frame, and aone-frame delay does not cause significant degradation of the image.When abrupt changes of brightness do occur, it takes time for the viewerto visually adjust to the image, so image errors due to the delay of theBL and INVy values do not stand out. See also U.S. patent applicationpublished as US 2009/0102783 A1 on Apr. 23, 2009, filed by Hwang et al.,incorporated herein by reference.

In FIG. 8, the one-frame delay is shown as block 804.

In some embodiments of the present invention, the delay is exploited todetermine the BL and INVy values from the ACC-corrected data for theprevious frame rather than the uncorrected data.

In the example of FIG. 8, the subscript “c” indicates the current frame.The RwGwBwWw data for the current frame are denoted RwGwBwWw_(c).

Scaler 150 scales the current frame's data RwGwBwWw_(c) with theINVy_(c−1) parameter determined from the previous frame. ACC block 320performs ACC correction and PCC metamer adjustment on the scaler'soutput RwGwBwWw_(c) for the current frame as described above inconnection with FIG. 3, 5, or 6, possibly as in Table 1. TheRwGwBwWw_(c) output of ACC block 320 can be processed as described above(e.g. by SPR block 510 in SPR displays, by output gamma 160, andpossibly in other manner) for display on screen 126.

The RwGwBwWw_(c) output of ACC/PCC block 320 is also provided toun-scale block 810 for “un-scaling”. The un-scaling can be performed bydividing the RwGwBwWw_(c) data by the INVy_(c−1) parameter determinedfrom the previous frame (or, equivalently, by multiplying theRwGwBwWw_(c) data by the BL_(c−1) parameter determined from the previousframe).

DBLC block 140 uses the RwGwBwWw_(c) data from un-scale block 810 todetermine the BL_(c) and INVy_(c) parameters, possibly in a conventionalmanner. These parameters are provided to scaler 150 and light source 110for use with the next frame “c+1”.

In some embodiments, the un-scaling is performed on the BL and INVyparameters rather than on RwGwBwWw. More particularly, un-scaling block810 is omitted. DBLC block 140 scales the thresholds (such as MAXCOL) bymultiplying the thresholds by INVy_(c−1), and uses the scaled thresholdsto determine the BL_(c) andINVy_(c values for the current frame from the un-scaled output RwGwBwWw)_(c) of ACC/PCC block 320. These values BL_(c) and INVy_(c) are then“un-scaled”, i.e. divided by the INVy_(c−1) (or, equivalently,multiplied by BL_(c−1)).

FIG. 9 shows an embodiment similar to FIG. 8, but the un-scaling 810 isperformed not on the RGBW data but rather on the difference dRw, dGw,dBw, dWw between the RwGwBwWw data output by ACC 320 and the RwGwBwWwdata output by circuit 130.2. The difference data dRwdGwdBwdWw aresmaller in size, and may need fewer bits and logic gates for theun-scaling. Adder 910 sums the un-scaled difference data dRwdGwdBwdWwwith the RwGwBwWw output of block 130.2, and the sum is provided to DBLCblock 140. The remaining processing is as described above in connectionwith FIG. 8.

Of note, if ACC block 320 is as in FIG. 5, then (dRw, dGw, dBw) areidentical to (dRi, dGi, dBi). In some embodiments, ACC block 320 is asin FIG. 6 (with metamer adjustment 604), but (dRw, dGw, dBw) are stillset to (dRi, dGi, dBi) to reduce the gate count.

The embodiments of FIGS. 8 and 9 are suitable for RGB displays. Forexample, ACC 320 may perform luminance correction for the RGB data. Theluminance correction may be accomplished, for example, by tables similarto ACC0 tables 220 (FIG. 2). In some embodiments, the corrected RGBluminance is in a linear color space, i.e. the output of ACC 320 can belinear RGB coordinates RwGwBw. The invention is not limited to RGB orany other set of primary colors except as defined by the appendedclaims.

Now possible metamer adjustments before scaling will be described. InLCDs, to provide high image quality with minimum output power BL, theRwGwBwWw coordinates of each pixel 406 should preferably be close toeach other. In some embodiments, in block 130.2, Ww is set to max(r, g,b), or to the average of r, g and b, or in some other way. See forexample the aforementioned U.S. patent application 2006/0244686 filed byHiggins et al. However, the ACC adjustments may frustrate the goal ofminimizing the output power BL or whatever other goals may have beenpursued by the metamer choice in conversion block 130.2. Therefore, insome embodiments, RGBW conversion 130.2 is followed by furtherACC-related metamer adjustment ACCMET 1010 (FIG. 10) which adjusts themetamers based on the anticipated ACC operation. For example, a metamermay be adjusted so that after the ACC the RwGwBwWw coordinates will beclose to each other.

Of note, equations (9) may require the values Rw, Gw, Bw to exceedMAXCOL and be as high as MAXCOL/M₀. For example, if b=0, then Ww=0; ifr=g=MAXCOL, then Rw=Gw=MAXCOL/M₀. For the sake of illustration, we willassume that M₀=M₁=½, i.e. the white subpixels are as bright as the red,green and blue subpixels. In this case, the Rw, Gw, and Bw values can beas high as 2*MAXCOL. The screen unit 120 accepts only colors whoselinear RwGwBwWw coordinates do not exceed MAXCOL. To display the othercolors, the backlight unit's power can be increased by a multiplicativefactor BL up to 1/M₀ (e.g. doubled). In this case, the RwGwBwWwcoordinates should be multiplied by INVy=1/BL. This multiplicationshould reduce the RwGwBwWw coordinates to the allowable range of 0 toMAXCOL inclusive. The maximum INVy value INVy_(max) required to forcethe RwGwBwWw coordinates into the allowable range determines the minimumBL value BL_(min). However, to save power, some embodiments do notincrease the backlight unit's power or they increase the backlightunit's power by a multiple smaller than BL_(min). Therefore, theRwGwBwWw data multiplied by INVy=1/BL may include coordinates outsidethe allowable range, and therefore scaler 150 may perform some gamutclamping to force the RwGwBwWw coordinates into the allowable range.Suitable gamut clamping techniques include hard clamping and other knowntechniques. See for example U.S. patent application published as no.2007/0279372 A1 published on Dec. 6, 2007, filed by Brown Elliott etal., entitled “MULTIPRIMARY COLOR DISPLAY WIITH DYNAMIC GAMUT MAPPING”,incorporated herein by reference.

FIG. 10 illustrates one non-SPR embodiment in which the output of block130.2 is provided to metamer adjustment circuit 1010 which adjusts theRwGwBwWw values as appropriate for the subsequent ACC operation in block320. In some embodiments, ACCMET 1010 reduces image distortion in gamutclamping. The RwGwBwWw values adjusted by ACCMET 1010 are provided toscaler 150. The scaler output RwGwBwWw is provided to ACC block 320. ACCblock 320 may be as in FIG. 3, 5, or 6. The Ww output of ACC 320 may bemultiplied by white gain WG using multiplier 520. Since no SPR is used,the pixel values Rw, Gw, Bw, Ww act as the subpixel values Rs, Gs, Bs,Ws, and are provided to output gamma 160.

DBLC 140 is coordinated with the white chrominance correction becausethe color correction performed by ACC 320 may change the peak subpixelvalues Rw, Gw, Bw, Ww and hence may affect the appropriate setting ofthe backlight unit's output power (specified by the BL parameter). Moreparticularly, the output of ACCMET 1010 is provided to ACC_DBLC circuit1020. ACC_DBLC 1020 performs a function similar to the ACC function ofblock 320 to estimate the ACC white chrominance correction. The estimateis taken into account by DBLC 140 in generating the BL and INVy values.One-frame delay may or may not be used, i.e. the BL and INVy valuesgenerated from the current frame may be used for the current frame orfor the next frame or for some subsequent frame.

FIG. 11 is identical to FIG. 10 except for the addition of SPR block 510at the output of ACC 320. The white gain adjustment by multiplier 520 isperformed on the Ws output of SPR 510. Output gamma 160 can be as inFIG. 3 or 5 for example. ACC 320 can be as described above in connectionwith FIG. 3, 5 or 6.

In some embodiments, ACCMET 1010 is omitted, but ACC_DBLC 1020 ispresent. In other embodiments, ACC_DBLC 1020 is omitted by ACCMET 1010is present. In some embodiments, the functions of ACCMET 1010 andACC_DBLC 1020 are combined in circuitry and are not clearly separable.

FIG. 12 shows one embodiment of ACCMET 1010. Table 2 below illustratespseudocode which can be used as simulation code for one embodiment ofthe ACC block 320 of FIG. 5. The simulation code is written in LUA.

The RGBW output of conversion block 130.2 is provided to Delta ACCPredictor 1210. ACC Predictor 1210 (lines ACM1-ACM27 in Table 2)predicts the white chrominance correction which will be performed by ACC320. The prediction is not always completely accurate but may be anapproximate estimate. For example, the predictor may use a table similarto ACC1 table 240 of FIG. 5 or 6, but with fewer bits for each entry toreduce the circuit size. Further, since ACCMET 1010 may change the whitecoordinate Ww, the corresponding chrominance correction computed basedon the changed coordinate may be different from the predicted correctioncomputed based on the Ww value input to ACCMET 1010.

In the embodiment of lines ACM3-ACM14, the predicted values are 7 bitswide, even though the RwGwBwWw coordinates are 12 or 14 bits wide. Thepredicted white value W_predict is a scaled value obtained as theproduct of Ww with the white gain factor (line ACM5) and with the INVyvalue INVy_(c−1) for the previous frame (line ACM6). The valueINVy_(c−1) is used by scaler 150 to scale the data for the currentframe. (Of note, the white gain factor and the predicted values areexpressed as integers equal to 256 times their actual values, sodivision by 256 is performed when needed.) The resulting W_predict valueis clamped to the maximum 7-bit value of 127 (line ACM7) and is providedto an ACC table similar to ACC1 table 240 but with fewer bits. In Table2, the ACC1 table 240 is shown as three tables accRmini, accGmini,accBmini (lines ACM8-ACM10). These tables provide the red, green andblue correction values Dr, Dg, Db. These values are divided by INVy(lines ACM11-ACM13) to undo the scaling, because the metamer adjustmentwill be performed on the un-scaled RwGwBwWw values output by block130.2.

Lines ACM16-ACM26 show another option for calculating Dr, Dg, Db. Inthis option, the predicted values are as wide as the RwGwBwWw values(i.e. 12 or 14 bits). The W-predict value is clamped to MAXCOL ratherthan being truncated to 7 bits. Tables accRdel, accGdel, accBdel areused instead of accRmini, accGmini, accBmini. In other respects, thisoption is similar to the option of lines ACM3-ACM14.

After calculation of Dr, Dg, Db by block 1210, metamer adjustments aremade consistent with the goal of metamer selection. In the example ofTable 2, the goal is assumed to reduce the output power of backlightsource 110. To achieve this goal, the maximum Ws value for each whitesubpixel should be about equal to the maximum of the Rs, Gs, Bs valuesfor the adjacent red, green and blue subpixels. In the embodiment ofTable 2, an attempt is made to force the ACC-adjusted Ww value for thepixel being processed to be about equal to the Rw, Gw, Bw values for thepixel. It is assumed that in conversion block 130.2, the Ww value wasdetermined as maximum of Rw, Gw, Bw values for the pixel. To gain someintuitive understanding, let us assume that M₀=M₁=½, Rw=Gw=Bw=Ww, andDr=Dg=Db, and in the absence of ACCMET 1010 the ACC block 320 wouldincrement Rw, Gw, Bw by a value D which is the common value of Dr, Dg,and Db. Then to make Rw=Gw=Bw=Ww at the output of ACC block 320, ACCMET1010 should decrement Rw, Gw, Bw by D/2, and should increment Ww by D/2.

The D/2 value is first determined in “negative adjustment” block 1220 asshown in lines ACM34-ACM45 (despite the “negative adjustment” name, D/2can be either positive or negative). The negative of the value D/2 islabeled “neg_adj” in Table 2. More particularly, as shown in linesACM35-ACM37, the predicted corrected values R_corr, G_corr, B_corr arecalculated as the sum of Rw, Gw, Bw and the respective predictedcorrections Dr, Dg, Db (the corrections can be either positive ornegative). The maximum of R_corr, G_corr, B_corr is determined in lineACM39. If the maximum equals the red corrected value, then neg_adj isset to −Dr/2. If the maximum is not equal to the red corrected value butis equal to the green corrected value, then neg_adj is set to −Dg/2. Ifthe maximum is not equal to the red or green corrected values but isequal to the blue corrected value, then neg_adj is set to −Db/2. Thenegative adjustment neg_adj is then added to Rw, Gw, Bw (linesACM55-ACM58). The sum is denoted as R_low, G_low, B_low. This sum couldbe the RGB coordinates of the new metamer unless the sum includes anegative component or the Ww adjustment would lead to overflow orunderflow.

The total value to be added to Rw, Gw, Bw output of block 130.2 isdenoted as RGB_adj (line ACM83), and will be added to Rw, Gw, Bw inlines ACM92-ACM94, and then hard-clamped to zero in lines ACM97-ACM99.The result will be the RGB coordinates of the new metamer.

RGB_adj is the sum of three values: neg_adj calculated by circuit 1220as described above, pos_adj1, and pos_adj2. The values pos_adj1 andpos_adj2 are calculated by positive adjustment circuit 1230 to avoidunderflow.

More particularly, pos_adj1 is introduced to correct R_low, G_low, B_lowfor underflow. Avoiding underflow (i.e. a negative value Rw, Gw, Bw orWw) is more important than avoiding overflow because the overflow can becorrected by scaling 150. If the minimum of R_low, G_low, B_low isnegative, then pos_adj1 is set to the negative of the minimum (linesACM59-ACM60). Otherwise, pos_adj1 is set to 0, i.e. no correction isneeded (line ACM62). Then R_low, G_low, B_low are incremented bypos_adj1 (lines ACM64-ACM66). The result is shown as R_low_pos,G_low_pos, B_low_pos.

The value pos_adj2 is introduced to avoid possible underflow in ACCblock 320. More particularly, in lines ACM69-ACM71, the ACC-adjusted RGBoutput of block 320 is estimated as the sum of (R_low_pos, G_low_pos,B_low_pos) and (Dr, Dg, Db). The sum is shown as (R_low_cc, G_low_cc,B_low_cc). If the minimum of these three values is negative, thenpos_adj2 is set to the negative of the minimum (lines ACM72-ACM74).Otherwise, pos_adj2 is set to zero (line ACM76).

Computation of RGB_adj (line ACM83) and subsequent processing areperformed by adjustment block 1240. In lines ACM84-ACM91, the valueRGB_adj is adjusted to reduce the possibility of underflow of the whitevalue Ww when the metamer adjustment is performed in line ACM95. Thewhite decrement W_adj is initially computed as RGB_adj/M₂ (line ACM84),but then is hard-clamped to the maximum of Ww (line ACM85), and thenRGB_adj is re-calculated as W_adj*M₂ (line ACM87).

Of note, lines ACM89-ACM90 show another embodiment without correction toavoid underflow. In either case, the new metamer values R, G, B, W arecomputed in lines ACM86, ACM92-ACM99 by adding RGB_adj to the RwGwBwoutputs of block 130.2 and subtracting W_adj from the Ww output. Thenthe new metamer's coordinates are hard clamped to the minimum of zero(lines ACM97-ACM100).

TABLE 2 ACCMET ACM1.   MINIACC = MINIACC or 1 ACM2.   if MINIACC == 1then ACM3.    W_predict = Ww        -- 14 bit W ACM4.   W_predict =math.floor(W_predict/(2{circumflex over ( )}(GAMBITS−7))) --shift right7 bits, use only the upper 7 bits of W ACM5.   W_predict =math.floor(W_predict*(WG+1)/256) --white gain can decrease white hereACM6.    W_predict = math.floor(W_predict*INVy/256)  --use old postscale value ACM7.   W_predict = math.min(W_predict,127)      --clamp itto 7 bits for looking up in the table ACM8.    Dr =accRmini[W_predict]   --RGB delta values are fetched ACM9.    Dg =accGmini[W_predict]   --these are 7bits (6 + sign bit) ACM10.   Db =accBmini[W_predict] ACM11.   Dr = math.floor(Dr*LEDy/4)  --multiply thedelta values by ACM12.   Dg = math.floor(Dg*LEDy/4)  --the inverse ofINVy, which is 2*LEDy ACM13.   Db = math.floor(Db*LEDy/4)  --/128 woulddo the 2*, but we need to multiply by 32 ACM14.        --to get into therange of the 14bit RGB values, leaving only /4 ACM15.  elseACM16.   W_predict = Ww        -- 14 bit W ACM17.   --W_predict =math.floor(W_predict/(2{circumflex over ( )}(GAMBITS−7))) --shift right7 bits, use only the upper 7 bits of W ACM18.   W_predict =math.floor(W_predict*(WG+1)/256) --white gain can decrease white hereACM19.   W_predict = math.floor(W_predict*INVy/256)  --use old postscale value ACM20.   W_predict = math.min(W_predict,MAXCOL)   --clamp itto MAXCOL bits for looking up in the table ACM21.   Dr =accRdel[W_predict] --RGB delta values are fetched ACM22.   Dg =accGdel[W_predict] ACM23.   Db = accBdel[W_predict] ACM24.   Dr =math.floor(Dr*LEDy/128)  --multiply the delta values by ACM25.   Dg =math.floor(Dg*LEDy/128)  --the inverse of INVy, which is 2*LEDyACM26.   Db = math.floor(Db*LEDy/128)  --/128 would do the 2*, but weneed to multiply by 32 ACM27.  end ACM28.  -- Lower power metameradjustment ACM29   ----------------------------------------------------------------------------------------------- ACM30.   -- Try to reduce the future power requirement of themetamer ACM31.   -- by finding the delta value that will affect the peakRGB value (power requirement) of the pixel ACM32   -- and then by usingthat value to push the metamer in the opposite direction ACM33.ACM34.   -- Perform a trial color correction to examine peak valuesACM35.    R_corr = Rw+Dr ACM36.    G_corr = Gw+Dg ACM37.    B_corr =Bw+Db ACM38.    min_RGB_corr = math.min(R_corr, G_corr, B_corr)ACM39.    max_RGB_corr = math.max(R_corr, G_corr, B_corr)ACM40.  LOW_POW= LOW_POW or 0 ACM41.   if LOW_POW ==1 then ACM42.     if max_RGB_corr == R_corr then ACM43.      neg_adj = −Dr/2ACM44.     elseif max_RGB_corr == G_corr then ACM45.      neg_adj =−Dg/2 ACM46.     elseif max_RGB_corr == B_corr then --this case is notlikely ACM47.      neg_adj = −Db/2 ACM48.     end ACM49.    elseACM50.     neg_adj = 0 ACM51.    end ACM52.  ... ACM53.   ... ACM54.  --Examine the proposed low-power metamer and see if it has gone negativeand by how much ACM55.    R_low = Rw + neg_adj ACM56.    G_low = Gw +neg_adj ACM57.    B_low = Bw + neg_adj ACM58.    minRGB_low =math.min(R_low, G_low, B_low) ACM59.    if minRGB_low < 0 thenACM60.     pos_adj1 = −minRGB_low -- reverse the negative sign and storeit as a positive adjustment ACM61.    else ACM62.     pos_adj1 = 0ACM63.    end ACM64.    R_low_pos = R_low + pos_adj1 -- push up the RGBcomponent so that low-power metamer isn't negative ACM65.    G_low_pos =G_low + pos_adj1 -- ACM66.    B_low_pos = B_low + pos_adj1 ACM67.   --Perform a trial color correction to see if it might go negative in thefuture and then do an optional prevention ACM68.   ------------------------------------------------------------------------------------------ ACM69.    R_low_cc = R_low_pos + Dr -- = Rw + Dr + neg_adj +pos_adj1 = R_corr + neg_adj + pos_adj1 ACM70.    G_low_cc = G_low_pos +Dg ACM71.    B_low_cc = B_low_pos + Db ACM72.    minRGB_low_cc =math.min(R_low_cc, G_low_cc, B_low_cc) ACM73.    if minRGB_low_cc < 0then ACM74.     pos_adj2 = −minRGB_low_cc -- reverse the sign and storeit as a further positive adjustment ACM75.    else      -- two positiveadjustment are kept separate on purpose ACM76.     pos_adj2 = 0   --ACM77.    end ACM78.  ... ACM79.  NEG_PREVENT = NEG_PREVENT or 0ACM80.    if NEG_PREVENT==1 then ACM81.    -- Calculate the preventativeadjustment values required for the RGB component and the W componentACM82.    -- and make sure W doesn't go negative ACM83.     RGB_adj =neg_adj + pos_adj1 + pos_adj2 ACM84.     W_adj = math.floor(M2_inv/32 *RGB_adj) -- M2_inv = M0*M1_inv; boosting the RGB reduces the WACM85.     W_adj = math.min (Ww, W_adj)        -- Do not reduce W to avalue lower than zero ACM86.     W = Ww − W_adj ACM87.     RGB_adj =math.floor(M2_reg/256 * W_adj) ACM88.   else ACM89.     RGB_adj =neg_adj      -- no negative prevention ACM90.     W_adj =math.floor(M2_inv/32 * neg_adj) ACM91.    end ACM92.    R_neg = Rw +RGB_adj  -- ACM93.    G_neg = Gw + RGB_adj  -- ACM94.    B_neg = Bw +RGB_adj  -- ACM95.    W_neg = Ww − W_adj ACM96. ACM97.    R =math.max(0, R_neg)  -- clamp any negatives to 0 ACM98.    G =math.max(0, G_neg) ACM99.    B = math.max(0, B_neg) ACM100.    W =math.max(0, W_neg) ACM101.  return R,G,B,W ACM102.  end END OF TABLE 2

Table 3 illustrates LUA code for another ACCMET embodiment. Thisembodiment corrects the Ww value while taking into account the actual,measured relationship between the white chromaticity and thechromaticities of the red, green and blue subpixels. More particularly,in the above Table 2 line ACM84, the white adjustment value W_adj iscalculated as RGB_adj/M2; then Ww is decremented by W_adj (line ACM95),and correspondingly Rw, Gw, Bw are each incremented by RGB_adj (linesACM92-ACM94). This computation assumes that a white subpixel'schromaticity is equal to the chromaticity provided by a red subpixel, agreen subpixel, and a blue subpixel together at equal values Rw=Gw=Bw.In an actual display, this assumption may be untrue. In the embodimentof Table 3, this assumption is abandoned. In lines AMT34-AMT36 of Table3, the white decrement is shown as Wm, and a white subpixel's value of 1corresponds in chromaticity to experimentally determined values for onered, one green and one blue subpixels. These values are shownrespectively as:

RWR_REG/128, GWR_REG/128, BWRREG/128

Therefore, when the Ww coordinate is decremented by Wm (line AMT37), thered, green and blue coordinates Rw, Gw, Bw are incremented, at linesAMT34-AMT36, by respective values shown as Wm*(RWR+1)/128,Wm*(GWR+1)/128, Wm*(BWR+1)/128. (The addition of 1 is done for properrounding in integer arithmetic.)

This technique of taking into account the relationship between the whitechromaticity and the red, green and blue chromaticities can be appliedto the embodiment of Table 2 or other embodiments. Of note, the Table 2embodiment is designed to reduce power consumption. The Table 3embodiment does not address power consumption. The Table 3 embodimentattempts to prevent the ACC 320 from generating negative Rw, Gw, Bw, Wwcoordinates. More precisely, the Table 3 embodiment attempts to preventACC 320 from generating negative Bw coordinates. The blue coordinatesare singled out because in many displays (including many LCDs) the whitesubpixels emit bluish light, and consequently the blue coordinates aremore likely to become negative in ACC than the red and greencoordinates. (Any negative coordinates output by ACC 320 can behard-clamped to zero.)

In more detail, the embodiment of Table 3 operates as follows. LinesAMT2-AMT10 are similar to lines ACM3-ACM13 of Table 2. The value W inTable 3 corresponds to W_predict in Table 2. The W value is nottruncated (in contrast with line ACM4), but is hard-clamped to themaximum of MAXCOL in line AMT4. In lines AMT11-AMT13, the coordinatesRw, Gw, Bw are incremented by Dr, Dg, Db respectively, and the resultsare denoted Rn, Gn, Bn. The values Rn, Gn, Bn are thus the red, greenand blue predicted outputs of ACC 320. The minimum of Rn, Gn, Bn iscomputed as Mn (line AMT15), and the negative of Mn is denoted as RGBm(line AMT18). The remaining steps of Table 3 are performed only if Mn isnegative (line AMT16).

At lines AMT24-AMT26, the values WBR_REG, GWR_REG, BWR_REG arecalculated from data stored in internal registers. At line ATM29, thewhite decrement Wm is calculated as RGBm/BWR_REG (or rather asRGBm*WBR_REG where WBR_REG is the inverse of BWR_REG). At linesAMT30-AMT32, the Wm decrement is hard-clamped to the maximum permissiblevalue of Ww (since Ww should not become negative when the decrement isapplied, at line AMT37). At lines AMT34-AMT36, the Rw, Gw, Bwcoordinates are incremented by values calculated from Wm as describedabove, and at line AMT37 Ww is decremented.

Lines AMT41-AMT52 illustrate another, simpler embodiment in which thered, green and blue coordinates are incremented by RGBm. Moreparticularly, the white decrement Wm is set to RGBm/M2 (see line AMT42),hard-clamped to the maximum of Ww (lines AMT43-AMT45), RGBm isrecomputed as Wm*M2 (lines AMT46) in case the hard clamp changes Wm, andthen the red, green and blue coordinates are incremented by RGBm (linesAMT48-AMT50). Ww is decremented by Wm in line AMT51. This embodimentdoes not take into account the chromaticity relationship between thewhite subpixels and the red, green and blue subpixels.

TABLE 3 ACCMET AMT1.  function Accmet(Rw,Gw,Bw,Ww) AMT2.   localW=math.floor(Ww*(WG+1)/256)    --perform white gain AMT3.   W =math.floor(W*INVy/256)    --perform part of the post scale AMT4.   W =math.min(W,MAXCOL)        --prevent OOG W AMT5.   local Dr =ACCInterp(W,accR256)  --perform the Delta ACC lookup AMT6.   local Dg =ACCInterp(W,accG256) AMT7.   local Db =ACCInterp(W,accB256)   --currently B is only delta    that can benegative AMT8.   Dr = math.floor(Dr*LEDy/128) --invert this intopre-(post scale) space AMT9.   Dg = math.floor(Dg*LEDy/128) AMT10.  Db =math.floor(Db*LEDy/128) AMT11.  local Rn=Rw+Dr        --perform theDelta ACC AMT12.  local Gn=Gw+Dg AMT13.  local Bn=Bw+Db AMT14.  localtest=0    --debugging only AMT15.    local Mn=math.min(Rn,Gn,Bn)  --findthe minimum of all three AMT16.  if Mn<0 then           --will any gonegative? AMT17.   test=255 --white dot indicates metamer AMT18.   localRGBm = − Mn AMT19. AMT20. --separate R,G,B metamer adjust AMT21.AMT22. RGB_SEP= RGB_SEP or 1 AMT23. WBR_REG= WBR_REG or 91 --White toblue metamer    ratio ; 8bit registers ; 127 is unity ; max value is 255representing    ratio of two AMT24. RWR_REG= RWR_REG or 131 --red towhite metamer ratio AMT25. GWR_REG= GWR_REG or 107 --green to whitemetamer ratio AMT26. BWR_REG= BWR_REG or 176 --blue to white metamerratio AMT27. AMT28.   if RGB_SEP==1 then AMT29.    local Wm= RGBm *(WBR_REG+1)/128    -- typically    convert the negative blue value towhite value (please note    WBR=1/BWR) AMT30.   if Ww−Wm < 0then    --don't let white value go negative AMT31.     test=190 --greyindicates full metamer out of reach AMT32.     Wm=Ww AMT33.   endAMT34.   Rw = Rw + (Wm * (RWR_REG+1)/128) -- white adjust value is   converted to red adjust value AMT35.   Gw = Gw + (Wm *(GWR_REG+1)/128) -- white adjust value is    converted to green adjustvalue AMT36.   Bw = Bw + (Wm * (BWR_REG+1)/128) -- white adjust value is   converted to blue adjust value AMT37.   Ww = Ww − Wm AMT38.AMT39. --combined RGB metamer adjust AMT40. AMT41.   else        --addthis to RGB to prevent that AMT42.   local Wm =math.floor(M2_inv*RGBm/32) --scale RGBm to make    Wm a metamerAMT43.   if Ww−Wm < 0 then  --don't let Wm go negative eitherAMT44.     test=190 --grey indicates full metamer out of reachAMT45.     Wm=Ww --the maximum metamer change we can do is thisAMT46.     RGBm = math.floor(M2_reg*Wm/256)  --scale Wm to    make RGBma metamer AMT47.   end AMT48.   Rw = Rw+RGBm      --generate the metamerAMT49.   Gw = Gw+RGBm AMT50.   Bw = Bw+RGBm AMT51.   Ww = Ww−WmAMT52.   end AMT53. AMT54.  end AMT55.  return Rw,Gw,Bw,Ww AMT56. endEND OF TABLE 3

FIG. 13 illustrates display circuitry according to another embodiment ofthe vector method. The data path following dither block 270 is as inFIG. 2 or 3. Output gamma 160 of FIG. 13 can be as in FIG. 3, with onlyACC0 tables 220, 230. The table 230 corrects the white luminance but notthe white chrominance. Tables 220, 230 can be constructed using knowntechniques. For fully saturated colors, when Wg=0, tables 220 ensurecorrect color display.

The white chrominance is corrected by input ACC block 1310 constructedas described below. Input ACC block 1310 receives sRGB dataR_(in)G_(in)B_(in) and outputs sRGB data “RGB” to provide correct whiteluminance for when R_(in)=G_(in)=B_(in). The RGB output of ACC block1310 is converted to linear RGB data “rgb” by input gamma block 130.1,and the rgb output of block 130.1 is converted to linear RGBW data Rw1,Gw1, Bw1, Ww1 by block 130.2. The blocks 130.1, 130.2 can be as in FIG.6 for example. Alternatively, input ACC 1310 can be combined with Inputgamma 130.1.

The input sRGB data R_(in)G_(in)B_(in) are also provided to input gammablock 130.1M which can be identical to block 130.1 of FIG. 6. The rgboutput of block 130.1 is converted to sRGB data Rw0, Gw0, Bw0, Ww0 byblock 130.2M which can be identical to block 130.2 except as describedbelow. The blocks 1301.M, 130.2M are designed to provide correct colordisplay for fully saturated colors. These blocks do not perform anycolor correction because the correct color display for fully saturatedcolors is achieved by means of ACC0 tables 220 as described above.

The RGBW outputs of blocks 130.2, 130.2M are supplied to interpolationcircuit 250. The select input of interpolation circuit 250 receives theSat output of block 260. Block 260 can be like in FIG. 2. For example,in some embodiments of FIG. 13:

Sat=Ww0/max(Rw0,Gw0,Bw0,1)

Alternatively, Sat can be generated as:

Sat=Ww1/max(Rw1,Gw1,Bw1,1)  (12)

Interpolation circuit 250 can be as in FIG. 2, and in particular it mayperform linear or non-linear interpolation. In the case of linearinterpolation, if Sat is between 0 and 1 inclusive, in some embodimentsthe output RwGwBwWw of interpolation circuit 250 is:

Rw=Sat*Rw1+(1−Sat)*Rw0

Gw=Sat*Gw1+(1−Sat)*Gw0

Bw=Sat*Bw1+(1−Sat)*Bw0

Ww=Sat*Bw1+(1−Sat)*Ww0  (13)

Thus, if the input sRGB data R_(in)G_(in)B_(in) define a white or nearlywhite color, then the Ww0 value will be high (e.g. equal to the maximumof R_(in), G_(in), B_(in)), so Sat will be close to 1, and henceRwGwBwWw will be close to the white-chrominance-corrected outputRw1Gw1Bw1Ww1 of block 130.2. For saturated colors, Sat will be close to0, and hence RwGwBwWw will be close to the uncorrected outputRw0Gw0Bw0Ww0 of block 130.2M.

Digital processing block 210 may include scaling and DBLC operations150, 140 as in FIG. 7 for example.

The input ACC tables 1310 for the red, green and blue data areconstructed after output gamma table 160 by providing sRGB dataR_(in)=G_(in)=B_(in) at different levels and measuring the white colordisplayed on screen 126. The tables 1310 are constructed to provide theRGB output such that the white color would be the ideal or near-idealcolor at each sRGB input level R_(in)=G_(in)=B_(in).

RGBW generation block 130.2M can be identical to block 130.2. In someembodiments however the block 130.2M generates low Ww0 (e.g. Ww0=0) ifthe input color is dark even if the input color is “grey” or close togrey, where “grey” denotes a color identical to white in chrominance.The reason for this construction is that humans are more perceptive tobrightness variations of dark colors, so quantization errors of darkcolors are particularly noticeable. If Ww0 is set to 0, then the RGBcoordinates Rw0, Gw0, Bw0 will be increased, the corresponding subpixelswill be brighter, and hence quantization errors will be less noticeable.Due to the selection of low Ww0 for dark colors, the output ofinterpolation block 250 will be close to the non-ACC-adjusted valueRw0Gw0Bw0Ww0.

Thus the Ww0 value is a function of brightness defined by the rgb outputof block 130.1M. If the rgb color is dark, the Ww0 function is zero ornear zero. If the rgb color is bright, the Ww0 function is the same asthe Ww1 function defined by block 130.2. As the rgb color changes fromdark to bright, the Ww0 function of block 130.2M can increase from 0 tothe Ww1 function gradually rather than abruptly to avoid artifacts.

FIG. 14 shows a slightly less gate-intensive embodiment in which theinterpolation 250 is performed before conversion to RGBW. Input ACC1310, input gamma 130.1 and 130.1M, output gamma 160, and block 210 canbe as in FIG. 13. The output of input gamma 130.1 is shown as r1g1b1,and the output of input gamma 130.1M is shown as r0g0b0. Block 260 cancompute the Sat parameter as:

Sat=min (r0,g0,b0)/max(r0,g0,b0,1)  (14)

Alternatively, Sat can be generated as:

Sat=min (r1,g1,b1)/max(r1,g1,b1,1)

The output rgb of interpolation circuit 250 can be:

r=Sat*r1+(1−Sat)*r0

g=Sat*g1+(1−Sat)*g0

b=Sat*b1+(1−Sat)*b0  (15)

Non-linear interpolation can also be used. The rgb output of block 250is converted to RGBW by block 130.2 which can be as in FIG. 7. Theoutput of block 130.2 is provided to digital processing block 210.

The invention is not limited to the embodiments described above. Forexample, the invention is applicable to displays other than RGB andRGBW. One example is RGBCW (Red, Green, Blue, Cyan, White) displays. Insuch displays, the white chrominance can be corrected by red, green,blue, and cyan subpixels. Also, the cyan chrominance may be corrected bygreen and blue subpixels. Other primary colors are possible.

The term “color coordinate” is sometimes used ambiguously herein todenote both a coordinate axis in a color space (e.g. R in RGB, or X inXYZ) and a value of the color coordinate (e.g. “Rw” in RGBW colorspace).

Some embodiments provide a method for processing image data fordisplaying an image by using a plurality of primary colors (e.g. RGBW orRGBCW or some other primary colors), the primary colors comprising aplurality of first primary colors (e.g. RGB) and one or more secondprimary colors (e.g. W).

The method comprises obtaining a color signal (e.g. RsGsBsWs in FIG. 2)representing one or more colors in the image, the color signalcomprising a representation of an amount of each said primary color inthe one or more colors. The amounts may be Rs, Gs, Bs, Ws for example.

The method further comprises obtaining a first signal (e.g. Rg0Gg0Bg0)incorporating a first correction of the color signal. The first signalis dependent on the amount of each of one or more of the first primarycolors in said representation. E.g. the first signal may depend on Rs,Gs, and Bs, or just on Rs or Bs, and may depend on additionalparameters, e.g. Ws.

The method comprises obtaining a second signal (e.g. Rg1Gg1Bg1)incorporating a second correction of the color signal, the second signalbeing independent of the amounts of the first primary colors in saidrepresentation but being dependent on one or more of the amounts of theone or more second primary colors in said representation. For example,in FIG. 2, Rg1Gg1Bg1 does not depend on Rs, Gs, Bs but depends on Ws.

The method comprises obtaining a relative-amount signal (e.g. Sat) whichrepresents an indication as to a relative amount, in saidrepresentation, of at least one of the one or more second primary colorsrelative to at least one of the first primary colors. For example, therelative amount can be the inverse of Sat since the inverse of Sat stillrepresents Sat hence represents the relative amount of Ws relative tothe maximum of Rs, Gs, Bs.

The method comprises obtaining, from the first and second signals andthe relative-amount signal, a combined-correction signal (e.g. RgGgBg)representing a combined correction which depends on the first and secondcorrections and on the relative amount.

In some embodiments, the first signal is independent of the amounts ofthe one or more second primary colors in said representation. Forexample, in FIG. 2, Rg0Gg0Bg0 is independent of Ws.

In some embodiments, the combined correction is closer to the firstcorrection if the relative amount is smaller. For example, in FIG. 2,RgGgBg is closer to Rg0Gg0Bg0 if Sat is smaller.

In some embodiments, the relative amount is an increasing function ofone or more of the amounts of the one or more second primary colors andis a decreasing function of one or more of the amounts of the firstprimary colors. For example, in FIG. 2, Sat is an increasing function ofWs and a decreasing function of each of Rs, Gs, Bs.

In some embodiments, the combined-correction signal defines the combinedcorrection as one or more grey values in a non-linear color space.

In some embodiments, the amounts of each color are values of colorcoordinates in a linear color space.

Some embodiments provide an apparatus for displaying an image, theapparatus comprising circuitry for correcting a color signalrepresenting one or more colors in the image, the color signalcomprising values of color coordinates (e.g. RsGsBsWs) of each saidcolor, the color coordinates being associated with a plurality ofprimary colors, the primary colors comprising a plurality of firstprimary colors (e.g. RsGsBs) and one or more second primary colors (e.g.Ws), the color coordinates of each color comprising first coordinatesassociated with the first primary colors and comprising one or moresecond coordinates associated with the one or more second primarycolors.

The circuitry comprises a first circuit (e.g. table 220 of FIG. 2) forgenerating a first signal (e.g. Rg0Gg0Bg0) incorporating a firstcorrection of the color signal, the first signal being dependent onvalues of one or more of the first coordinates in the color signal.

The circuitry comprises a second circuit (e.g. 240) for generating asecond signal incorporating a second correction of the color signal, thesecond signal being independent of the values of the first coordinatesin the color signal but being dependent on values of one or more of theone or more second coordinates in the color signal.

The circuitry comprises an interpolation circuit (e.g. 250) forgenerating a combined-correction signal by interpolation from the firstand second signals based on an indication of a relative amount of atleast one of the one or more second primary colors relative to at leastone of the first primary colors.

Some embodiments provide an apparatus for displaying an image by using aplurality of primary colors (e.g. RGBW), the primary colors comprising aplurality of first primary colors (e.g. RGB) and one or more secondprimary colors (e.g. W), the apparatus comprising circuitry forcorrecting a color signal representing one or more colors in the image,the color signal comprising values of one or more color coordinates ofeach color (e.g. R_(in)G_(in)B_(in) in FIGS. 13, 14).

The circuitry comprises a first data path for processing the colorsignal; and a second data path for processing the color signal; whereinthe first and second data paths comprise shared circuitry. For example,in FIG. 13, the first data path may include the entire circuit exceptblocks 1310, 130.1, 130.2; the second data path may include the entirecircuit except blocks 130.1M, 130.2M, 260. In FIG. 14, the first datapath may include the entire circuit except blocks 1310, 130.1; thesecond data path may include the entire circuit except blocks 130.1M,260.

The first data path comprises a first color correction circuit (e.g.130.1M, or a combination of 130.1M and 160) comprising a separate colorcorrection circuit (e.g. a separate table in input gamma 130.1M) forcorrecting each value of a plurality of first sets of the colorcoordinates. For example, the first sets may be a set {R_(in)}, a set{G_(in)}, and a set {B_(in)}. In some embodiments, each first setcomprises one or more but less than all of the color coordinates of eachcolor. In some embodiments, the separate color correction circuits ofthe first data path are operable to correct the color signalrepresenting any one of the first primary colors to a predefined displayquality. For example, in FIG. 13, the tables 130.1M are operable tocorrect the color signal representing any one of the red, green and blueprimaries to a predefined quality.

The second data path comprises a second color correction circuit (e.g.1310) comprising a separate color correction circuit for correcting eachvalue of a plurality of second sets of the color coordinates, eachsecond set comprising one or more but less than all of the colorcoordinates of each color, the second data path being operable tocorrect the color signal representing at least one second primary colorto a predefined display quality. E.g., in FIGS. 13-14, the input ACC1310 is operable to correct the color signal representing the whitecolor to a desired quality.

The shared circuitry comprises an interpolation circuit (e.g. 250) forgenerating a combined-correction signal by interpolation from a signalobtained using the first color correction circuit and a signal obtainedusing the second color correction circuit, the interpolation being basedon an indication (e.g. Sat) of a relative amount of at least one of theone or more second primary colors relative to at least one of the firstprimary colors in the one or more colors.

In some embodiments, the shared circuitry comprises a third colorcorrection circuit (e.g. output gamma 160) connected downstream of theinterpolation circuit, the shared circuit being for correcting amountsof the primary colors (e.g. amounts Rs, Gs, Bs).

Some embodiments provide a method for processing image data fordisplaying images in a display's screen unit comprising a plurality ofsubpixels, wherein each subpixel has a luminance depending on thesubpixel's state defined using a corresponding subpixel value. Theimages are displayed using a light source providing light to the screenunit. The method comprises (1) obtaining image signals which representeach image using color coordinates; and (2) generating the subpixelvalues from the image signals. The operation (2) comprises, for eachimage IM1 of one or more of the images (e.g. for the image “c” in FIG. 8or 9):

(2A) Generating a scaling-information signal (e.g. INVy_(c−1) and/orBL_(c−1)) which represents scaling information which comprisesinformation on an output power to be generated by the light source indisplaying the image IM1 and on how the image signals representing theimage IM1 are to be scaled to match the output power of the lightsource, the scaling information being dependent on one or more of theimages;

(2B) Scaling values of the color coordinates of the image IM1 inaccordance with the scaling information to generate a scaled-imagesignal;

(2C) Obtaining a color-correction signal (e.g. RwGwBwWw_(c))representing a color correction for the scaled-image signal for theimage IM1, the color-correction signal being used to provide at leastone of the subpixel values.

In some embodiments, the scaling information in the scaling-informationsignal for the image IM1 is obtained from at least partialrepresentation of the color correction for at least one image (e.g. apreceding image, e.g. image “c−1”). “At least partial representation ofthe color correction” may be the output RwGwBwWw_(c) of ACC 320 as inFIG. 8, or the output dRw, dGw, dBw, dWw of ACC 320 as in FIG. 9, orsome other representation.

Of note, scaling may involve multiplication of the values of the colorcoordinates by a fixed value such as INVy, but may also involvenon-uniform scaling, i.e. some type of adjustment of the values of thecolor coordinates to the output power. For example, the colorcoordinates may be in a nonlinear color space.

In some embodiments, generating the scaling-information signal for theimage IM1 comprises generating an un-scale signal (e.g. the input ofcircuit 140 in FIG. 8 or 9) at least partially indicative of an adjustedcolor correction for the at least one image, the adjusted colorcorrection being the color correction adjusted for an output power levelindependent of the scaling information for the at least one image (e.g.the output power level can be the reference level corresponding to theun-scaled output of circuit 130.2, in other words the reference levelcorresponding to BL=1), the un-scale signal being generated from thescaling information of the at least one image and from an at leastpartial representation of color correction for the at least one image.

Some embodiments provide an apparatus for processing image data fordisplaying images in a display's screen unit comprising a plurality ofsubpixels, wherein each subpixel is to have a luminance depending on thesubpixel's state defined using a corresponding subpixel value, whereinthe images are to be displayed using a light source for providing lightto the screen unit. The apparatus comprises a circuit for obtainingimage signals which represent images using color coordinates, and forgenerating the subpixel values from the image signals. The circuitcomprises:

(A) a dynamic-light-source-control (DLSC) circuit for determiningscaling information for the images (e.g. the circuit 810, 140, 804 ofFIG. 8, or 810, 910, 140, 804 of FIG. 9), the scaling informationcomprising information on an output power to be generated by the lightsource in displaying the images and on how the image signalsrepresenting the images are to be scaled to match the output power ofthe light source;

(B) a scaler circuit (e.g. 150) for scaling values of the colorcoordinates of the images in accordance with the scaling informationprovided by the DLSC circuit and generating scaled values of the colorcoordinates;

(C) a color-correction circuit (e.g. 320) for correcting the scaledvalues and generating corrected scaled values of the color coordinates,the corrected scaled values being used to provide the subpixel values.

In some embodiments, the DLSC circuit is responsive to thecolor-correction circuit to generate the scaling information based on atleast partial information on the corrected scaled values.

The display may or may not be SPR type.

Some embodiments provide a method for processing image data fordisplaying an image. The image comprises obtaining a color signalcomprising color coordinates' values (e.g. RwGwBwWw in FIG. 6) of one ormore colors in the image. For each said color (i.e. for each of the oneor more colors recited above, and thus possibly for each color in theimage or for each of a subset of colors in the image), the followingoperations are performed. From the color signal, a corrected-colorsignal is obtained (e.g. RwGwBw output of adder 310 and Ww output ofcircuit 210) which comprises corrected values of the color coordinates,the corrected values incorporating a correction of one or more of thecolor coordinates' values of said color for a display's transfercharacteristic (e.g. the correction may involve correction of Rw, Gw, Bwbut not Ww, but may involve other corrections). From the corrected-colorsignal, an adjustment signal is generated (e.g. RGB_adj_pcc in Table 1)incorporating information on an adjustment to be performed on thecorrected values for the color. The corrected-color signal is modifiedwith the adjustment signal to generate an adjusted-corrected-colorsignal (e.g. RwGwBwWw output of PCC metamer adjustment 604) comprisingadjusted corrected values of the color coordinates of the color, thecolor coordinates comprising at least one color coordinate whoseadjusted corrected value is greater than the corrected value andcomprising at least one other color coordinate whose adjusted correctedvalue is smaller than the corrected value (e.g. RwGwBw may be increasedwhile Ww is decreased, or vice versa).

In some embodiments, each color coordinate is associated with arespective primary color, the primary colors comprise a plurality offirst primary colors (e.g. RGB) and one or more second primary colors(e.g. W), each second primary color being representable as a mixture oftwo or more first primary colors, and the color coordinates comprisefirst color coordinates associated with the first primary colors andcomprise one or more second color coordinates associated with the one ormore second primary colors;

The color coordinates comprise at least one first color coordinate whoseadjusted corrected value is greater than the corrected value andcomprise at least one second color coordinate whose adjusted correctedvalue is smaller than the corrected value, or the color coordinatescomprise at least one second color coordinate whose adjusted correctedvalue is greater than the corrected value and comprise at least onefirst color coordinate whose adjusted corrected value is smaller thanthe corrected value.

Some embodiments provide an apparatus for processing image data fordisplaying an image, the apparatus comprising a circuit for processing acolor signal comprising color coordinates' values of one or more colorsin the image. The circuit comprises:

a color-correction circuit (e.g. 240, 310 in FIG. 6) for obtaining fromthe color coordinates' values, for each said color, corrected values ofthe color coordinates, the corrected values incorporating a correctionof one or more of the color coordinates' values of said color for adisplay's transfer characteristic;

an adjustment circuit (e.g. 604) for determining if an adjustment to beperformed on the corrected values, and if the adjustment is to beperformed, then modifying the corrected values to generate adjustedcorrected values of the color coordinates of the color, the modifyingcomprising increasing the corrected value of at least one of the colorcoordinates and decreasing the corrected value of at least one othercolor coordinate.

Some embodiments provide a method for processing image data fordisplaying one or more images in a display's screen unit comprising aplurality of subpixels, wherein each subpixel has a luminance dependingon the subpixel's state defined using a corresponding subpixel value.The images are displayed using a light source providing light to thescreen unit. The method comprises, for each image IM1 of one or more ofthe images, obtaining an image signal comprising color coordinates'values (e.g. output of block 130.2) of one or more colors in the imageIM1. The method further comprises generating a scaling-informationsignal (e.g. INVy, BL) which represents scaling information whichcomprises information on an output power to be generated by the lightsource in displaying the image IM1 and on how the image signalrepresenting the image IM1 is to be scaled to match the output power ofthe light source, the scaling information being dependent on one or moreof the images. The method further comprises adjusting the image signal(e.g. by ACCMET 1010) to obtain an adjusted image signal whichrepresents the image IM1 using adjusted values of the color coordinates.The method further comprises scaling the adjusted values of the colorcoordinates of the image IM1 in accordance with the scaling informationto generate a scaled-image signal (e.g. output of scaler 150) comprisingscaled values of the color coordinates. The method further comprisesobtaining, from the scaled-image signal, a color-correction signal (e.g.output of ACC 320) representing a color correction for the scaled-imagesignal for the image IM1, the color correction being a correction forthe display's transfer characteristic, the color-correction signal beingused to provide at least one of the subpixel values.

Further, for at least one color in the image IM1, the color coordinatescomprise at least one color coordinate whose adjusted value is greaterthan the corrected value by a first amount (e.g. RGB_adj in Table 2) andcomprise at least one other color coordinate whose adjusted value issmaller than the corrected value by a second amount corresponding to thefirst amount, the first and second amounts depending on the scalinginformation for at least one image.

In some embodiments, the color-correction signal comprises correctedscaled values of the color coordinates; and the adjusting operationcomprises obtaining an estimate (e.g. Dr, Dg, Db) indicative of a changeof at least one corrected scaled value relative to the correspondingscaled value, and comprises adjusting the corresponding value in theimage signal such that if the corrected scaled value is greater than thecorresponding scaled value, then the corresponding adjusted value issmaller than the corresponding value in the image signal, and if thecorrected scaled value is smaller than the corresponding scaled value,then the corresponding adjusted value is greater than the correspondingvalue in the image signal.

APPENDIX Additional Methods for Building ACC Tables

Measure Output Gamma:

When characterizing a display it is common to measure the gamma. Thegamma is the luminance response curve of the display to a linear ramp ofvoltage or a linear ramp of digital input values. To best match thehuman eye for indoor viewing, the output gamma response curve shouldapproximate a power curve with an exponent of 2.2. The brightness of thedisplay is measured at a series of input points to see if it followsthis curve. This data can be used to adjust resistor ladders in thedisplay to make it more closely match the correct curve, or it can beused to generate a digital table that corrects values to compensate forthe display.

Input and Output Gamma:

The 2.2 gamma response of a display means that all data sent to adisplay has a non-linear assumption. An input value half way through therange does not map to a brightness that is radiometrically half waybetween the darkest and brightest value. It is difficult to doarithmetic on these non-linear values and get the correct results.Because of this, we usually convert all color values to linear values atthe beginning of our “gamma pipeline”, do all the processing in theselinear values, and convert them back to non-linear values at the end.These conversions can be done with LUTs (Look-Up-Tables) or acomputational module or a combination. Conversion to linear values atthe beginning of the gamma pipeline is usually done with the “sRGBassumption”, a standard specifying the non-linear quantizationassumption in most small computer image files. Converting back tonon-linear at the end then could simply be the inverse of the sRGB inputgamma However, if the output conversion back to non-linear is theinverse of the measured display gamma instead, then the outputconversion will also compensate for any irregularities in the display.With the output gamma completely canceled, the only remaining gamma willbe a perfect sRGB curve.

Gamma on Each Channel:

The gamma response curve of each channel (R G B and W in our case) inthe display is measured separately, so there are four different outputgamma tables. This is done in ACC, e.g. in the output gamma 160 ofFIG. 1. The same tables can be used as the ACC0 Luminance correctiontables 220 in FIGS. 2 and 3, among others. On other multi-primarydisplays there may be different or additional output tables. Typicallythe same input gamma curve is used on all the input channels since thisis an assumption about the source data, not the output display. We takethe luminance measurements from R G B and W and condition each of themusing a custom program called Gamma_Curve. This program interpolatesmissing values when a complete dataset is not available, using a cubicinterpolation algorithm. It converts floating point measurement valuesinto integer values that are normalized to the range of values that thehardware can store in LUTs. If necessary, this program can also generatean input gamma curve that is the inverse of the conditioned output gammacurve. (This will be used in the discussion below).

Chromaticity Measurements:

The luminosity measurements are taken with a colorimeter which can alsomeasure the chromaticity of the display. It is quite common to measurejust the chromaticity of each channel at the brightest setting. Fromthese chromaticity values and the brightest luminance value on eachchannel it is possible to generate a matrix that converts from displayvalues to CIE XYZ tri-stimulus values. From XYZ it is possible toconvert into almost any other color space, such as sRGB, Lab, Luv,YCbCr, etc.

In the case of a display with three primaries, such as an RGB display,the conversion matrix can be inverted and used to convert XYZ colorsinto values suitable for use on the particular display. This can allowyou to more accurately display colors on a display, or to display colorsthat come from sources other than RGB files. In the case of an RGBWdisplay, or any multi-primary display with more than 3 primaries, theconversion matrix from RGBW to XYZ is not square and cannot be inverted.This is the same as stating that there four unknowns but only threeequations. A single solution is not always possible, and conversion toRGBW has to be done with an algorithm as in (9).

Chromaticity Ramp Measurements:

As impressive as linear algebra and matrix arithmetic sounds, theconversion matrixes developed above have a huge assumption: The matrixis built from measurements of just a few bright values, and assumes thatthe chromaticity remains constant all the way down through dark valuesto black. If measurements of the chromaticity are taken at allbrightness values, you will find that this assumption is often notcorrect: The chrominance values drift as the brightness of the displaychanges. Leakage is one problem that you will see in the dark colors:The chromaticity of all the primaries will appear to change to that ofgrey-white as they approach black. This is expected and cannot beavoided. Changes in the chromaticity of the brighter colors, however isundesirable. This can cause visual defects even in an RGB display, suchas mid-level grays looking yellow or blue compared to the full on white.In the case of DBLC (Dynamic Back Light Control), changes in thechromaticity with input level can cause the “full on white” to changecolor as the backlight is turned up and down while the LCD is turneddown and up to compensate. The goal of our ACC is to compensate for thischange in chromaticity with input value. The measurements of luminanceand chromaticity for a range of input values on all the primaries allowsus to build ACC tables to do this.

Conditioning the Measurements:

The measurements taken are luminance and chrominance for a ramp of inputvalues for R G B and W separately. Typically a display will accept 256different input values. It is not necessary to measure all of theseinput values, as long as the darkest and brightest and sufficient valuesbetween are measured. Normally we take 32 evenly spaced measurements ofeach primary, plus a 33rd brightest possible value. There is always acertain amount of noise in these measurements so the data is filtered toaverage out some of this noise. A simple box filter is currently used.After filtering, we interpolate the values to produce a dataset thatdoes have 256 values. This allows using values in the normal range asthe device and the normal tables, such as the input gamma table, whendoing simulations. Currently we are using a cubic interpolation thatgenerates points smoothly between the measured values.

Our colorimeter records measurements as CIE xyY values. The x,y valuesare the chrominance, the capital Y is the luminance. CIE xyY values arelinear values by design, however, our input samples are effectivelyspaced non-linearly. The luminance curves on a perfect display wouldlook like 2.2 power curves if plotted on simple graph paper. If plottedon the correct power-axis graph paper they would look like straightlines. In our gamma pipeline we use an sRGB input gamma LUT to linearizeinput vales. The same thing could be done with the measured data tobring the samples into a linear space. The result is a sparse dataset,so it is also interpolated to the full bit width of the interior of thegamma pipeline. Typically this stretches the input values to 12 bitswide. At this point the measurement values are still stored as floatingpoint.

However, using the same sRGB input gamma that is used in the pipelinemay not be the correct one to use. Instead, we take the output gammafrom each channel and invert those into input gamma tables. (Using thesame program described above that generated the output gamma tables).Since the output gamma tables were generated from the luminance data,this process should convert the luminance data to a straight line on allchannels. This is rarely precisely achieved because we use input andoutput gamma tables that have been rounded to integers. This moreclosely simulates the behavior of our digital hardware, but introduces alittle quantization noise.

When this linearization process is done to the chrominance data, itchanges the spacing of the sample points, concentrating them more on thedark than on the bright end of a ramp. In either non-linear or linearspace, we would like the chromaticity to be constant for any inputvalue, a horizontal line. But the characteristics of the LCD shutterscause the chromaticity to vary. In addition leakage through the LCDdisplay causes the chromaticity to turn towards grey-white at somepoint, as mentioned above. One of our data conditioning steps is to tryto detect where this leakage starts to dominate and linearly interpolatethe chrominance values down from there. This prevents our ACC correctionfrom attempting to correct for leakage in dark colors.

Converting Any Input Color to CIE XYZ:

The result of the data conditioning steps is a set of tables which,given a linear color value, can look up the CIE xyY value that would beproduced by the hardware. The measured values are only from separateramps of R G B and W, but we can convert xyY values into XYZ valueswhich can be summed together. This allows us to predict color values forany of the 17 million colors that the display can produce. We alsomeasure a few other ramps, one with the R G B values increasingtogether, another with R G B and W increasing together. Comparing thesemeasured combined values with the results of summing the separate R G Band W values together gives us some confidence that the theoreticalsumming of XYZ values correctly simulates the summing of colorsoptically on the display.

ACC Table Construction:

A CIE XYZ graph of the chromaticity of a black to white ramp (R G B andW all increasing from 0 to max together) should be a straight line, theline of grays. Due to chromaticity drift as the voltage changes in anLCD cell, this is rarely true. A ramp of all the white ramp values isgenerated and converted to CIE XYZ using the table method as describedabove. This produces the uncorrected gray ramp. The goal of our ACCprocess is to change the RGBW values slightly to bring the color backonto a desired straight line of grays. It is possible to start with adesired white point in CIE XYZ space and generate a series of grayvalues that linearly increase from black to white. 256 values are simplyinterpolated between black (all zeros) and the white point. Using themeasured 100% white point of the display (R G B and W on full) producesACC tables with the smallest possible values. The purpose of ACC is tolinearize the line of grays, but it could also be used to change thecolor temperature of the display by choosing a desirable white pointsuch as D65. If this is done, the size of the final ACC table values mayincrease requiring more storage or even becoming too large for practicaluse.

For each uncorrected gray ramp value we now have a desired gray value.The uncorrected values are in RGBW and in CIE XYZ, the desired gray rampvalues are only in CIE XYZ. We have a simple process for converting RGBWto CIE XYZ but as mentioned above, there is no simple way to do thereverse. So we cannot simply convert the desired grey ramp values toRGBW. However, because we can do the reverse, if we stumbled upon a goodcombination of RGB&W, we can verify whether or not it is close to thedesired grey ramp value. With this we can use numerical solvingtechniques to search for the closest RGBW combination that produces thedesired grey ramp XYZ value. There are many numerical solving methods inthe mathematical literature. Most of them are designed for solvingcontinuous functions and many of them require that you know thederivatives of the functions. A secondary goal of many of these methodsis to find the minimum value in the fewest possible steps. We have adiscontinuous function, RGBW values can only change in integer steps,and we do not have a function for the rate of change in XYZ as RGBWchanges. However, we do not care how many steps it takes to find asolution, since we can run this process once and use it to build a tablethat is accessed rapidly later. So a brute force method is perfectlyadequate, and on a modern PC it turns out this only takes a few secondsanyway.

To numerically find the RGBW set that most closely produces a desiredCIE XYZ value, we start with an initial guess of the RGBW values. Onereasonable initial guess would be the uncorrected RGBW value thatcorresponds to the desired grey ramp value. A better initial guess wouldbe the solution from a nearby point. In the current code, a combinationof the two is used: an uncorrected value in the middle of the range isused as the initial guess. From this it takes many iterations to findthe best solution. Then this first solution is used as the initial guessto find the next element up or down in the table. From this it takesfewer iterations to find the minimum. Each successive solution is usedas the guess for the next solution up or down until the table iscompleted.

Starting with an initial guess, we simply change the R G B and W valuesby +/−1, convert to CIE XYZ, and compare that to the desired CIE XYZvalue. If the ‘distance’ to the desired value has decreased, we arecloser to the solution. Converting the CIE XYZ values to CIE Labco-ordinates and calculating the distances in that color space would beone way to generate a distance that is generally considered meaningful.(CIE Lab was designed so that a Pythagorean distance of 1.0 would be a“just noticeable distance” to a standard observer). In the spirit ofdoing a brute force approach, we are simply using the Pythagoreandistance in CIE XYZ space to calculate a distance. Although distances inXYZ space are generally not considered to be meaningful, it is areasonable practical method of getting a rough estimate of the closenessof our solution. Changing the R G B and W values by +/−1 and moving inthat direction when the distance decreases causes our solver to “walkdown hill” to a minimum distance and the closest RGBW set that match thedesired gray ramp value. Currently our ACC methods are all indexed bythe W value, so we typically hold the W values constant and only allowthe solver to walk down hill on the R G and B axes.

On the displays that we have seen so far, the chromaticity of the whiteramps have typically had too much blue. So the corrected RGBW valueshave decreased blue values and slightly increased red and green. Becausegreen carries more of the luminance than blue, the result has been aslight increase in the luminance curve of a gray ramp. One solution tothis would be to allow the solver to change the W value to bring theluminance back to the original level. Because our tables are indexed byW our preferred embodiment avoids this. Another solution to theincreased luminance after ACC is to characterize the new luminancecurve, and change the output gamma table for W to compensate.

Using the tables from measurements to convert linear RGBW to XYZ canonly convert values that are in-gamut. If the solver follows the slopeoff the edge of one or more of the tables, this probably means thatthere is no good solution. This typically happens on the dark or brightends of the ramp where there is little room to move away from the lineof grays. In these cases, we stay just inside the gamut and find thebest solution possible.

The result of running the solver on every gray ramp value is a table ofcorrected RGBW values that more closely produce the desired line ofgrays when displayed on the hardware. This table is an intermediatestep, it is used to generate other tables that are used in severaldifferent ACC methods.

Vector ACC Method:

In the vector ACC method, we interpolate between a grey ramp correctedRGBW value and the uncorrected RGBW value, based on the saturation ofthe color. This interpolation is done after output gamma, so the tableof corrected values developed above could be used if the results areimmediately run through the output gamma tables. However, those twosteps can be convolved together resulting in one table for R G and B.This is the ACC1 Chroma correction table 240 of FIG. 2. There would bethree tables, but since they are all indexed by the same Ws value, theycan be combined into one table with a single address decoder. If lookedat as separate tables, they would look similar to the R G and B ACC0luminance correction tables.

Delta ACC Method:

In the delta ACC method, we calculate the RGB distance between thecorrected and uncorrected RGBW grey ramp values. These are stored in adelta ACC chroma correction table 240 of FIG. 3. Some conditioning isdone on the delta table before using it. Typically the delta valuesshould approach zero at the dark and bright ends of the ramp. Butbecause the solver error increases near the ends, the delta table oftenbecomes noisy in these areas. We often clamp the values to zero or forcethem to ramp down to zero near the ends to remove this noise. The restof the delta ACC table can also be noisy, so we filter it to smooth outdiscontinuities that might produce visible effects in images. At runtime, the delta ACC table is indexed by the Ws value and added to everyR G and B value in the pipeline.

Simulating the Effects of the Delta ACC tables.

After building the tables, we burn them into EEPROMs in display hardwareand measure the effect on grey scale ramps, among other images. Beforethe hardware is ready it is possible for us to use the tables tosimulate the expected effects, at least on a black to white ramp, andsee if the ACC tables will linearize the chromaticity. The first methodused was a short-cut. A ramp of input values is run through the inputgamma table. In a grey scale ramp, the resulting R G B and W values allramp together, so simulating the conversion (9) was unnecessary. The Wvalue is used to index the delta ACC table and the deltas are added tothe R G and B. The tables generate above are used to convert this to CIEXYZ, then converted to the CIE xyY values that the colorimeter willproduce. These are plotted as predictions and compared against themeasured values. The results have been a very good match between theoryand practice.

The above method produced CIE XYZ values directly from linear RGBWvalues. This is the short-cut because it did not take the output gammatables into account. This was considered reasonable since the outputgamma and the display measurements are supposed to be inverses of eachother. This inversion is done with the luminance values, and might nothave the correct effect on chromaticity. To investigate this, we did asecond more complete simulation. A ramp of input value is run throughthe input gamma table to linearize them. The W value is used to indexthe delta ACC tables to get values to add to the linear R G and Bvalues. These plus the W are run through the 4 (R G B and W) outputgamma tables. The result values which are in the same non-linear spaceas the original measurements. So these values can be used to index theconditioned measurements (before linearization). These measurements arein CIE xyY which have to be converted to CIE XYZ so that they can besummed together. Then the sum is converted back to xyY for plotting andcomparison to the short-cut and to the measured data. This also resultedin a good match between theory, practice and practicality.

The invention is not limited to the embodiments described above. Otherembodiments and variations are within the scope of the invention, asdefined by the appended claims.

1. A method for processing image data for displaying an image by using aplurality of primary colors, the primary colors comprising a pluralityof first primary colors and one or more second primary colors, themethod comprising: obtaining a color signal representing one or morecolors in the image, the color signal comprising a representation of anamount of each said primary color in the one or more colors; obtaining afirst signal incorporating a first correction of the color signal, thefirst correction being to correct for a display's transfercharacteristic, the first signal being dependent on the amount of eachof one or more of the first primary colors in said representation;obtaining a second signal incorporating a second correction of the colorsignal, the first correction being to correct for the display's transfercharacteristic, the second signal being independent of the amounts ofthe first primary colors in said representation but being dependent onone or more of the amounts of the one or more second primary colors insaid representation; obtaining a relative-amount signal which representsan indication as to a relative amount, in said representation, of atleast one of the one or more second primary colors relative to at leastone of the first primary colors; from the first and second signals andthe relative-amount signal, obtaining a combined-correction signalrepresenting a combined correction which depends on the first and secondcorrections and on the relative amount.
 2. The method of claim 1 whereinthe first signal is independent of the amounts of the one or more secondprimary colors in said representation.
 3. The method of claim 1 whereindepending on the relative amount, the combined correction is equal tothe first correction, to the second correction, or to a value betweenthe first and second corrections, the combined correction being closerto the first correction if the relative amount is smaller.
 4. The methodof claim 1 wherein the combined correction is a result of linearinterpolation of the first and second corrections.
 5. The method ofclaim 1 wherein the relative amount is an increasing function of one ormore of the amounts of the one or more second primary colors and is adecreasing function of one or more of the amounts of the first primarycolors.
 6. The method of claim 1 further comprising using the combinedcorrection signal to display the image.
 7. The method of claim 1 whereinthe combined-correction signal defines the combined correction as one ormore grey values in a non-linear color space.
 8. The method of claim 5wherein the amounts of each color are values of color coordinates in alinear color space.
 9. The method of claim 1 wherein at least somecorrections of each second primary color to correct for the display'stransfer characteristic are operable to be performed by correcting twoor more of the first primary colors.
 10. An apparatus for displaying animage, the apparatus comprising circuitry for correcting, for adisplay's transfer characteristic, a color signal representing one ormore colors in the image, the color signal comprising values of colorcoordinates of each said color, the color coordinates being associatedwith a plurality of primary colors, the primary colors comprising aplurality of first primary colors and one or more second primary colors,the color coordinates of each color comprising first coordinatesassociated with the first primary colors and comprising one or moresecond coordinates associated with the one or more second primarycolors, the circuitry comprising: a first circuit for generating a firstsignal incorporating a first correction of the color signal, the firstcorrection being to correct for the display's transfer characteristic,the first signal being dependent on values of one or more of the firstcoordinates in the color signal; a second circuit for generating asecond signal incorporating a second correction of the color signal, thesecond correction being to correct for the display's transfercharacteristic, the second signal being independent of values of thefirst coordinates in the color signal but being dependent on values ofone or more of the one or more second coordinates in the color signal;an interpolation circuit for generating a combined-correction signal byinterpolation from the first and second signals based on an indicationof a relative amount of at least one of the one or more second primarycolors relative to at least one of the first primary colors.
 11. Theapparatus of claim 10 wherein the first signal is independent of thevalues of the one or more second coordinates in the color signal. 12.The apparatus of claim 10 wherein the interpolation is linear.
 13. Theapparatus of claim 10 wherein the relative amount is an increasingfunction of values of one or more of the one or more second coordinatesand is a decreasing function of values of one or more of the firstcoordinates.
 14. The apparatus of claim 10 wherein thecombined-correction signal defines the combined correction as one ormore grey values in a non-linear color space.
 15. The apparatus of claim10 wherein the color coordinates of each color are in a linear colorspace.
 16. The apparatus of claim 1 wherein each second primary color isrepresentable as a mixture of two or more of the first primary colors.17. An apparatus for displaying an image by using a plurality of primarycolors, the primary colors comprising a plurality of first primarycolors and one or more second primary colors, the apparatus comprisingcircuitry for correcting, for a display's transfer characteristic, acolor signal representing one or more colors in the image, the colorsignal comprising values of one or more color coordinates of each color,the circuitry comprising: a first data path for processing the colorsignal; and a second data path for processing the color signal; whereinthe first and second data paths comprise shared circuitry; wherein thefirst data path comprises a first color correction circuit comprising aseparate color correction circuit for correcting, for the transfercharacteristic, each value of a color coordinate of a plurality of firstsets of the color coordinates, each first set comprising one or more butless than all of the color coordinates of each color, the separate colorcorrection circuits of the first data path being operable to correct,for the transfer characteristic, the color signal representing any oneof the first primary colors to a predefined display quality; wherein thesecond data path comprises a second color correction circuit comprisinga separate color correction circuit for correcting, for the transfercharacteristic, each value of a color coordinate of a plurality ofsecond sets of the color coordinates, each second set comprising one ormore but less than all of the color coordinates of each color, thesecond data path being operable to correct, for the transfercharacteristic, the color signal representing at least one secondprimary color to a predefined display quality; wherein the sharedcircuitry comprises an interpolation circuit for generating acombined-correction signal by interpolation from a signal obtained usingthe first color correction circuit and a signal obtained using thesecond color correction circuit, the interpolation being based on anindication of a relative amount of at least one of the one or moresecond primary colors relative to at least one of the first primarycolors in the one or more colors.
 18. The apparatus of claim 17 whereineach of said first and second sets consists of a single colorcoordinate.
 19. The apparatus of claim 17 wherein the shared circuitrycomprises a third color correction circuit connected downstream of theinterpolation circuit, the shared circuit being for correcting amountsof the primary colors for the transfer characteristic.
 20. The apparatusof claim 19 wherein the shared circuitry further comprises a subpixelrendering circuit connected between the interpolation circuit and thethird color correction circuit.
 21. The apparatus of claim 19 whereinthe shared circuitry further comprises a control circuit connectedbetween the interpolation circuit and the third color correction circuitfor determining an output power of a light source used to display animage, the output power in displaying the image being dynamicallydetermined from one or more images displayed by the apparatus, the lightsource being for providing light to subpixels controlled by signalswhich are generated by the first and second data paths and which dependon the output power.
 22. The apparatus of claim 17 wherein the sharedcircuitry further comprises a subpixel rendering circuit connecteddownstream of the interpolation circuit.
 23. The apparatus of claim 17wherein the shared circuitry further comprises a control circuitconnected downstream of the interpolation circuit for determining anoutput power of a light source used to display an image, the outputpower in displaying the image being dynamically determined from one ormore images displayed by the apparatus, the light source being forproviding light to subpixels controlled by signals which are generatedby the first and second data paths and which depend on the output power.